Thanks, all. Weird - I've used the 3M strips as well, but they don't stick very well to the foam (for me, at least).
Anyhow, glad to hear the Auralux tabs work. I do like mounting them on card - a couple times I've then wrapped the square in a funky cloth, like tie dye. I wonder if that negatively impacts the absorption (?)
I've been considering the rockwool/wood frame option for a while - is it difficult to make? I'm not particularly handy. I did make three so-called "BBC" diffusers a couple of years ago. I read about making them in one of the pro audio mags. They made it sound so easy. Actually, it WAS easy, but it took many many hours when all the measuring, cutting, gluing, and painting was done.
The real killer for me is getting bass traps to stick on a wall/ceiling corner, especially when the ceiling is somewhat textured (not sure what you'd call it, looks sort of like plaster). Obviously flat is better, but still, I've tried everything.
Actually, that raises the question of whether the tabs will work on my ceiling with the panels, given that the ceiling isn't perfectly flat (?).
I'll post a picture of the ceiling when I get home and I'd love to hear your thoughts on the matter. Another issue is that an air shaft is behind the ceiling above my work station so any kind of hardware is out. I know, poor planning, but rearranging my entire studio is not an option (well, I suppose it could be, but... Ugh).
Thanks again for the helpful replies.