It seems like I make frames all the time for family, friends and neighbors. I made one just last week out of old barn wood for a neighbor. It measured 24" x 30" inside for a "glued together jig saw puzzle" she has.
But, I must mention that I use a table saw and not everyone has that available to them.
I don't do much fret work anymore but, when I did, I usually made my own frames if I felt like the project really needed one.
Rog