Just keep in mind that if you use rigid fiberglass, acoustical cotton, foam, etc. - they'll act as broadband absorbers - not just bass traps. For just trapping broadband BASS, you need to construct resonant panel traps with absorbant material inside. If you want to go the cheaper route (and easier) you can also just cover the absorbant material with a thick paper to emulate somewhat the performance of FRK 703 or 705 so it will reflect the mids/highs and still pass the bass for absorbtion.
If you WANT or can tolerate broadband trapping with all that 3" already up, 4" 3lb rigid fiberglass or Acoustical Cotton is the best material to work. The foam (IMO) just doesn't get down deep enough - especially if you know you're working with bass heavy material AND you have a small room where you need the most you can get out of every square foot of coverage.
Also, if you really want the room nice and tight, you'll likely need more than 1 trap. In small rooms like that, think about at a minimum, 2'x4' straddling the corners at 45 degrees at the wall/wall/ceiling junction. Floor to ceiling will be better.
The ultimate is to cut the 4" (or whatever) into 8 triangles from a 2x4 sheet and stack them on top of each other so the whole corner is filled - not just 4" spaced out from the corner. Realistically, 4" spanning the corner will give you 80%+ of the tightness and control of a solid corner.
