Treatments labeled as "acoustic panels" will usually have no effect in the bass range. You want something actually sold as a "bass trap".
You're on the right route with building your own. From that site, try the
Roxul Rockboard 60, cut into half and then diagonally, so you get 4 triangles which are 2 feet long on the short sides, and 2 inches thick. The $49 gets you 6 sheets by 4 triangles by 2 inches is: 4 feet thick. So you'd need $200 worth to do the 2 rear corners (really you only need to do 2 corners, as you're trying to break up standing waves between a pair of walls, break the pair, break the waves) from floor to an 8 foot ceiling.
If you want to cover the diagonal between the walls with their acoustic panels, that'll be a good use for them. But you'll need a custom size being 2'10" wide to cover the long side of the triangle.EDIT: I just found their absorption coefficients for the 4" thick panels. Just as I figured, they're good down to 250 Hz, but fall off quickly after that. If you do want to do the custom one to cover the rockwool, just go with the 1" thick and request that they be made with an open back (even though they don't usually do that for the less than 4" panels).
ADDITIONAL: Their 96"x34"x1", open backed, panels run $212 each. You'll probably just want to stretch your own fabric over a frame. The panel is just for looks anyway, because the rockwool is doing all the work.
