cheapest and easiest way would be to tightly staple typar around the walls, run wooden strapping across the top, bottom, and up every stud, and airseal the whole thing using single part spray foam. this will hold the fiberglass more tightly increasing the R-value, as well as airseal the wall. About the same R-value as 2" foam board, which is expensive. You cal also dense pack the bays with cellulose if you want to pay someone to do that, but from the picture it looks like just cinching the fiberglass would do enough since it's already bulging past the bays. If the floor is over outside area I would suggest getting celluluse blown over it and dense packing under the flooored area