I was thinking of a fairly large box, too big to stuff into any engine compartment I'm familiar with. I'm thinking a standard medium mover's box (18x18x16) And cutting off the 18x18 side so you have 2.25 square feet of open area. More cross-section area equates to less air flow per unit area and less chance the caked ash will be held onto the filter by the air flow.

The intake on my truck is a rubber hose that is tucked in front of the wheel well. The when well is plastic so cutting a hole to allow a hose to pass should be an issue. The hose would come out the top of the wheel well and up onto the hood where the large box filter would be mounted on blocks filter side down. Duct tape and cargo straps should hold it down well enough.

Made out of plywood and with a little streamlining the box should ride at any reasonable speed. A cardboard box would be more delicate and might require you to drive slower. Then again with ash limiting visibility driving slow might be wise.

Filterwise I'm thinking pantyhose material might be better than a furnace filter. Furnace filters are 'depth' filters and tend to trap the crud they filter out. That's good for a house where you simply replace them but in this roll you want something that will drop the ash if you give it a whack.

I would try two layers of pantyhose material and drop to one if it seemed too tight.