so, buy a ManualD or ASHRAE Fundamentals, study to learn how big of a return you need, what size plenums are necessary, weld together an angle iron stand, attach some sheet metal, run the return duct. Buy 2 pr aviation grade tin snips --
BTW, what is the easiest way to drop a gas tank from a 1991 Toyota? does it have a pump in that tank? does it have a filter in the tank? can I just put it up on stands?