Thanks for the replies, everyone!
I'm afraid crate-making is outside our skill set, but it may be helpful to others!
We need something approximately 24x18x15 - not exactly, of course, but that's what we got based on their measurements. They are both relatively large, full grown, male cats. We got carriers that were easily big enough for common sense, but probably not big enough for the airline. I find the need for a huge carrier odd, given that cats feel more comfortable in smaller spaces (makes them feel safe). But, mine not to wonder why...