Relep,
Here is what I settled on. I mostly frame unusual aspect ratios because I crop all my images to look their very best. Then I frame them myself and standardize my materials as much as possible. Aluminum sectional frames....always black, 4 ply white alpha cellulose "conservation" mat board and acid free 3/16 foam core for backing. I've ordered and keep on hand at least 4 sections of 16, 18, 22, 24, 26, 28 and 30 inch aluminum frame pieces. I decide how large to make the print, then add from 2 to 3.5" for borders and see what even number frame size works considering my frame sections. Example: Printed image dimension = 12x19.5 I'll then choose a frame size of either 16x24 (borders = 2.0 x 2.25) or 18x24 (borders = 3.0x3.25). I used to order frames individually like this for each print that I sold, but it took more time and added framing section shipping expense to every sale. So I opted for keeping pieces in hand at set, 2" increment lengths. Then I individually cut my foam core and mat to the frame size and custom cut the mat opening as needed. Finally, my local glass shop sells me custom cut glass at my desired size for $10 apiece.
Brad