OK, here is what I'm looking for (click on the thumbnail for a big image)

I need one sized to fit that hole.

If it were me, I'd take some measurements and go to a Lowes. They should have one that fits.

Tried that already

Must have been an unusual bottle. Looks like it's time to break out the carpentry skills and install a mount for a standard bottle of your choice -- don't skimp.

Yea, I was just trying to not rebuild everything at once

I bet that was for one of the old "home" size halon units. They were smaller like that.

Cut out the neck holder with a dremmel and attach a mount in the existing hole. That way you can use the existing strap as a secondary retention device.

Dry powder fire extinguishers simply aren't all that good for vehicle fires. Unless the powder can get to all of the fire directly your less likely to extinguish it. It is also weak because it doesn't cool the base of the fire and it doesn't do much to slow the spread and buy time unless the surface is one where the powder can build up on.

Halon and/or foam would be my choice, assuming a perfect world. Halon does a great job of getting around tangles of hoses and wire to get to the fire. Halon has excellent knockdown power. The Halon replacements aren't as good but they are still head and shoulders over any other type for knockdown of a flame front. Foam does pretty well itself but dry powder is, in comparison, pretty lame. Yes, the powder goes everywhere, but not in sufficient quantities to do much good if you can't hit the fire directly.

The second point is that dry powder tends to simply fall away from where it would do some good. I have seen automotive fires where there were pretty substantial mounds of powder on the ground after they hauled away the burned out hulk away. They must have shot a dozen of the small dry chemical units, seemingly every extinguisher in a block of apartments, at what was a rather small fuel fire under the hood. But they couldn't get a straight shot and what they sprayed simply fell off and piled up under the car.

Which is an unfortunate characteristic of the agent. It doesn't stick well to much of anything. At least it doesn't until time to clean it up. It's unwillingness to cling means that unless the area is flat you can't use it to slow or eliminate the the spread of the fire. With foam you can coat and wet even the vertical surfaces around a fire. This can be a used to good effect on a fire that may be too big to put out to buy yourself time. Time to get people out, flee, or for help to arrive. Working from the outside in is a sound tactic.

People who are impressed by dry chemical extinguishers power to put out a fuel fire have usually used it on a standard pan-type training fire. Against a horizontal pool of fuel, where you can get a square shot, it works well.

Take a tenth that amount of fuel and dribble it down a vertical surface and try to put it out and there is a god chance you run out of extinguisher before you run out of fire. The powder keeps falling off so you have to corral the fire. If you can work your way bottom to top and get all the flame before your empty your golden. If not the last lick of flame relights the whole thing.

This is also how you extinguish a jet of fuel. Very impressive killing a 30' column of fire with a hand-held unit. Not so hard if you know the trick and practice. Wear suitable protective gear because failure to kill all the flame at close range means your going to be very close to a huge flashback.

I wouldn't give the electrical system on a vehicle a second thought. Your talking 12 to 24 volts and even fresh water is okay. In marine systems seawater is considered safe around such low voltages. The fire is always more of a hazard than the danger you may get a tingle. This may change as electric vehicles become common.

All that said most vehicles, if they carry a fire extinguisher at all, carry a small ABC type dry powder unit. First they chose a type that is not efficient, then they compound the error by selecting the smallest unit possible. A 2.5 pound unit is barely suitable for putting out a lit Zippo lighter IMHO. A trained and experienced firefighter might do some good with it but it is simply too small to do much good in the hands of someone who is uncertain what to do and how.

My philosophy is that if your going to carry a fire extinguisher carry one adapted to the job and get the largest one that is practical. There are no prizes for second place. ABC type extinguishers are inherently inefficient on many fires so you need biggest you can manage. Go big. A ten or twenty pounds unit isn't too big.

Don't be fooled into worrying that a larger extinguisher will make a bigger mess. It can, but only if you keep the lever squeezed. If the fire goes out before the powder is gone your not obligated to keep shooting. Far, far better to run out of fire before you run out of agent.

I've seen halon used in car fires at the track & I've watched a dry chemical unit used on a motor home fire on the interstate. The performance difference (to my untrained eye) was very significant.

Several decades ago I used to work at "Big 3" automotive proving ground in Michigan. While there I took a driver safety training course. When they talked about car fires that had just one specific bit of advice:

Before you get out of the car pull the hood release from inside the car.

They said that an engine fire will typically damage the engine-end of the hood release cable preventing fire crews from being able to unlatch and open the hood, which really limits their access when extinguishing the fire.

That's great advice & something most folks wouldn't think of in advance!