I think you're right Evan. The glass cabinets I have already look the part, but they're a touch flimsy, and rattle if moved. One is 23 inches wide, the other approximately 13 inches wide, both with an internal depth of approximately 11 inches.
Like Glenn, I figure an internal depth of 16 inches will do the job, without 'intruding' too much on the room they'll be in. This should allow larger 1/48th scale models, such as the B-25 and B-26, maybe even the B-17 if it's not on a display base, as well as 1/32nd scale models such as the Hunter and Tornado, and future builds, which won't fit the existing cabinets.
I reckon one around 36 x 16 inches, and another around 24 x 16 inches should do for now, and, with a bit of luck, they might cost less for both than it would for one 'shop bought' item.