Check with Sig Mfg. or someone here that sells there stuff. They still offered one last I knew, and I'm pretty sure it's 4". It's from one of the old Berkeley kits if I remember correctly.
Right...they still offer it, and it's only $2.00! (Brodak also has it for $2.39)
Downside is it's only 3-1/4". They also carry a short "horse collar" type, 360 degree ring for the same price. It tapers down slightly from the front radius, but it is 3-3/4".
I'd look to the fiberglass guys. They have most sizes.