I have a friend that has a TR4A with the surrey top. After looking at his front windscreen, I think Triumph used the same one on the TR6. When I went to the surrey, I replaced the windshield capping with the one used on the TR4 that is used for surreys. It fit perfect, even the predrilled holes on the capping lined up with my windscreen. I purchased the surrey off of ebay and installed the seal that came with it. I will have to check the one that TRF sells and see if its any better. I noticed the water comes in right at the ends of the windscreen capping.
Dale:
The plastic may work for you, but I don't want to replace rotted carpet every couple years. Of course if you don't have carpet in the car, then that's not a problem!