Simplest way? Either custom mats and framing, or cropping everything to standard sizes.
$30 isn't a bad price for a preassembled 20x24 frame with glazing.
Granted, the plexi it comes with is probably completely worthless, but the price is decent if the moulding is decent quality and the build seems solid.
Since I try to compose in the camera without cropping later, most of my prints are in 2:3 aspect ratio.
I buy precut 10 X 15 or 12 X 18 mats at matcutter.com and frame them in 16 X 20 (for 10 X 15) or 18 X 24 (for 12 X 18) frames from Michaels Art Supply.
I print my own photos on 11 X 17 or 13 X 19 paper and mount using archival corners and linen tape.
If I have to crop, I still print as close to those dimensions as possible and custom cut my own mat for those frame sizes.
