I had this problem -- the bracket for my main power panel needed a CT with .75 inch opening, but thin enough to fit into it.
Here's what I found (and did)...
There's a 750 CT that fits the service panel bracket perfectly. It's from Aim Dynamics. 303-772-9100 (ask for Joe or Victor @ 141 or 140)
Magnelab has stuff that will work, too, but I'm just a small fry. I'm sure they'd be happy to sell me box loads (but I just need two).
B