What I have done( in an old grainery that I converted into a shop). I cut a piece of plywood to fit the frame of the walk in door and mounted two 20" box fans with furnance filters and material from old bedsheets to keep the little insects out(bear in in mind it's a small shop). I had to leave the rollup door open some for airflow,but, many have said the results were great. You may have to sand and buff. But for me and the money I save, it is worth it.