Being an old guy with old ways, I'm partial to leather.
I've found that the 10X10X3 size is about right. If you make it much bigger (or deeper) you'll find that suff gets kinda lost down in there.
Things I require:
Good, heavy leather.
A wide/longish cover flap.
2 Straps to secure the flap.
Yep.... good leather saddle bags are expensive.
You can get the "ebay
type" rough-out bags perty cheap. But just know ahead of time that they are cheap for a reason.
So...if you're going to put them to any kind of heavy use, you'll have to re-stitch them with some good thread and do some re-inforcing of the bottoms, right from the get-go. The type of leather they are made from is the worse there is...and won't stand much weather (or abuse) as-is. So you might also give some thought to rubbing a bunch of shoe-grease (or mink oil) into them before subjecting them to rain, snow and/or a lot of hot sun during the summer.
In fact.... If you go that route, I'd say that buying two sets wouldn't be a bad idea. Maybe that way you could just remove the existing stitching, unfold everything and use the pieces to build one set made of 2-ply leather. That would also provide an opertunity to add a divder or something if you wanted to.
You can buy a hand-stitcher for leather and heavy canvas. It's called an awl and they usualy come pre-boxed with a good bit of heavy thread.
They are a bit slow but are quite usable....if you aint in a hurry.
Hope this helps.