Some GI's used 100 mph tape (aka duct tape) to secure knife sheaths to the shoulder straps of their load bearing equipment. Looks cool, but if the sheath is upside down (looks even cooler) it is real easy to accidentally unsnap the strap and lose your blade. But even if right side up, you only have your knife (or Leatherman) with you when you are wearing your (in this case) pack. So, you set up camp, stash the pack against a tree, then every time you need the Leatherman to do something you have to go back to the pack for it. Morebetter on you instead of the pack...

I've had a small 3" fixed blade knife on a pack strap for years. It does have a sheath that was designed for upside down carry. I've found it to be a very convenient way to carry this blade. I always have at least a SAK in my pocket, so I'm not totally screwed if I were to somehow lose it.

To lash it down, I used 1/8" bungee type cord, and did kind of a crossing pattern like shoelaces. It's very secure with a bit of give because of the stretchy cord. It's sold by the foot. I got it at REI, but I'm sure it's available elsewhere.

For something like this, depending on the backpack strap I think the constrictor knot would be really ideal. However, I would definitely have a direct line to the sheath just as a backup keeper, in case the lashing knots failed or loosened up. However, with the constrictor knot that's unlikely.
Here's a good site on the constrictor knot.

Jason

Redondancy ! <img src="/images/graemlins/cool.gif" alt="" />

I lashed an OutdoorWedge II to the left strap of my backpack. The special locking feature of the plastic sheath seems positive enough to carry it upside down.

But it is only an addition to a folding knife + a Leatherman PST II on my belt + another smaller folding knife in my pant's pocket + a Cold Steel SRK in a side pocket of the backpack (next outing, I will try to fix this one horizontally, on the pack waist belt).

All items on my pant's belt (or in pant's pocket) are not easily accessible, as the pack's waist belt more or less covers them.
OTOH (I mean the right one... <img src="/images/graemlins/grin.gif" alt="" />), I can have my Wedge II out of its sheath and ready to cut in a second.

I am still partial to using duct tape to secure an extra knife (fighting blade) to my combat harness. As the OldBaldGuy stated: it does you no good if it isn't on you & I tend to wear my harness after getting to & setting up camp. I still carry my Buck 110 & Leatherman on my belt.
Keep in mind, don't use duct tape unless you either never remove the knife sheath OR you place some paper or something between the harness and the duct tape. The duct tape leaves glue behind and things get messy after that.

Make sure you dummy cord it.

During my 1+ year in Vietnam (in combat, in the jungles, etc.) I kept an AF Suvival Knife attached UPSIDE DOWN to my pack's shoulder strap via duct tape until I came across some velcro straps. They were the cat's meow for this purpose. I still keep that same knife (in a replacement sheath) velcro strapped to my pack strap. One piece comes around and across the knife handle itself secuing it with something besides the flimsy leather and snap originally provided. I backpacked much of the Appalachian Trail, the Florida Trail, and made numerous hunting, camping and back-country fishing trips with that knife in that same position and I've never had a problem. I'd recommend a velcro strap at least 3/4" wide - and 1" would be better. They are available at many Internet sights, sporting goods stores and even big box stores like Walmart.

Something like this

Thanks for the help, from everyone. I am using a Camelbak MULE backpack. I haven't actually received it yet (bought is used on ebay) when I get it and attach the sheath I'll get a digital photo and show what I did. Again, thanks!