Unless you've got a huge dog then I'd recommend the 1" wide one, I originally only did the 1.5" wide ones because that is the depth of the actual Tx itself and it looked a nice neat job, however, what the fook would I know about it, I just make the collars, I'm definitely no Terrierman!
I would strongly recommend that a cable tie is used around the flap when in use (they cost about £2 for a 100), not because I don't have confidence in my work, but because if a flap can be opened to put the Tx in, then murphys law says that it will open when you least want it to! So either a cable tie or a