I use a regular filter on mine. I buy one the correct width and long enough to cover all the louvers. I sort of crease the long sides so that the filter will fit into the space where the old hog hair filter was. I have not had any problem with them.

Alternatively, you can measure the narrower width between the flanges and place the filter there. Use duct tape to hold it in place, as you won't have the tight fit of the flange holding it.

Pay attention to the arrow showing the proper airflow direction.

I was advised to do this by a qualified manufactured home maintenance company.

EDIT: You will want to use the cheap filters, not the expensive ones. The expensive ones can compromise the air flow.

Cathyyg, the air flow problem was what I was really concerned. I'll look for filters immediately. And of course no house is complete without duct tape! Even the cheap ones have to be better than the hog's hair ones.

Yeah, I wish I could use the expensive ones, or even better, add an electronic allergen air filter to the furnace. But it is not practicable with my furnace configuration.