If possible I would go from floor to ceiling. I would build 2x4 and stack them.

Corners are the most efficient place for bass absorption. 2" will absorb little lower freq energy. You really need a LOT of a fiber glass to slow down and absorb bass energy. I have 14" of 703 in the corner. I would have them stacked if the wall didn't slant in. Depending on placement I would say a box of 6 2x4' x2" in the corner is good for .5-1db of absorption 60hz and below, that might be really optimistic too.

You've mentioned how your bass output is "modest" I bet you will be surprised how much boost and suckout are in your room. I highly recommend getting a calibrated mic and measuring the room. It will also help you really get your sub integrated into the system/room.

I use this mic https://www.minidsp.com/products/acoustic-measurement/umik-1 $75
and REW https://www.roomeqwizard.com/ Free / Donation.