For now, foam - wedges and basic corner treatment.
I know I'd get better results building my own but, realistically, that just isn't going to happen - and should it ever happen, well, all the better.
Seeing how even just putting back the curtains on the walls had audible results, anything is better than nothing. Plus a friend uses the same stuff in his studio, and I was quite surprised with how much the acoustics had improved over the last time I'd been there.