I would be tempted to add a bit of hose/pipe to the existing intake before the filter and to route this to a simple box with one end cut out and a rough filter cloth, pantyhose?, covering the opening.

The box could be rough cut from plywood and nailed together, caulked and/or sealed with foil tape. But a simple cardboard box might get it for a time. The box shouldn't be under any great stress in use. I would sit the box on blocks on my hood, and tape down the assembly with duct tape.

One of the benefits of driving a beater is that you can modify and tape stuff to it without ruining the look.

I would arrange the box so the filter end was down so falling ash doesn't rest on the filter. Given the large area of rough filter the air flow would be quite slow through the filter even at high throttle. So the ash would tend to fall off the filter face and get swept away by the slipstream instead of clogging the filter. If it did get clogged it would be easy enough to stop the truck, get out, and give the box a few whacks so the ash falls off.