i have been working in sizes that fit ikea frames. i do a fair amount on 16x20 canvas. ikea has the VIRSERUM frame that is 15.75 x 19.75 and only costs $13. i use my circular saw and skim 2 sides to open it up to fit the canvas and use 1/4'' offset clips to secure canvas to frame.
for my abstract work on hardwood cradled panels, i use blick unfinished premier wood panels. they are quite nice and well priced and don't need to be framed.