Tim; Nice job on the inserts. The red looks good. May have to try that after I run out of black next time. I use a slant jig on my tablesaw to cut the coves in pieces of stock that I square up on my jointer and then plane to final dimension so that I'm 100% sure they are square. I find that about 4 feet is a handy length to start out with. I have a ton of 1"x1" square hardwood stock that was used to sticker lumber for kiln drying, it's kinda rough, but it cleans up nicely at 7/8 square.
Nowdays I leave the mitered spanish cedar liners in the boxes too, and use them to cover the edges of the insert, it's easier to glue on the fabric if you don't have to worry about covering the edges. Also the cigar store dude told me they "just don't look right" without them. I took his word for it, he deals with these things for a living. They also help to keep the boxes from warping as they dry out after being removed from the humidor. I hope you're not using spray adhesive to hold the fabric down. Tried that, got a couple back that didn't hold.
I also found that it's much better to cut a insert out of some sort of thin wood and glue the strips to it, rather than trying to glue them to each other. Seems that no matter how carefully you square your stock, when you glue 8 to 12 pieces side to side, some cumulative error creeps in and they don't want to lay flat. Besides, if you glue them to a backer square, you can use spacers between them to space them out and eliminate the gaps at the edge. (I cut up usps flat rate envelopes into strips the same height as my inserts, then trim them off with a disk sander) I try to have enough space to leave a double thickness on the outside edges now, it seems to look more finished for some reason. Anyway, just a few observations from having done a bunch of these, YMMV.