Don't tell the MG crowd on this forum, but I used some cheap MGB stainless center caps. Drilled a hole in the center, then mounted the Triumph centers using t-nuts. Cap was a bit loose on the 2.5" diameter center hole, so I cut a short piece of 2.5" pvc pipe and tapped it into the wheel center to give a smaller id. The pvc isn't very elegant, but it was cheap and the caps aren't moving. Better but more expensive way would be to get something turned out of aluminum but I'm in no hurry.
Using the MGB caps there is very little space left on the back wheels but it still worked out. I think there may be other center cap options out there but for now this works - and I think the dimensions look about right.
Randy