How can I unbend a bent furnace filter slot?
It sounds as if the slot is OK at least on one side of the bend, or perhaps both. If so you can make a shaper out of a piece of wood whose thickness is cut to the width of the slot.

Insert the wood in the good section of the slot and force it through the bent section. You may even be able to tap it sideways with a hammer.

If the wood is too soft to bent the metal back, try a harder piece of wood or even a piece of metal if you can find one of the right size. A piece of wood layered to the right thickness with a thin piece of metal toward the bent side of the track would also work.