GP,

the two most important functions of the iso booth will be to record vocals
and to record a cranked 4x12 guitar cabinet.

For a cranked guitar cabinet you need substantial absorption at low
frequencies. Thin materials won't do that.

would getting some of these RPG "modex corners" actually help?

RPG Modex bass traps are great, if expensive, but they're not appropriate
for this application. Modex traps are available tuned to different frequency
ranges, and those ranges are better suited for larger rooms. What you need
is broadband absorption that works well down to the lowest frequencies, not
just over a narrow range.

i could almost see myself raising up the guitar cabinet and then
"surrounding" the perimeter of the guitar cab with these modex things.

Modex traps are meant to be stacked in corners, not placed out in the room.
If you get EQ magazine, the current (January) issue has a review of my
company's MiniTraps. The reviewer specifically mentioned how successful
MiniTraps are for exactly what you describe.

--Ethan