I was shooting some high magnification macro work yesterday fooling with a macro lens - extension tube - 2X-L teleconverter - camera combination. The 2X-L seemed to perform fairly well based on what I saw in the viewfinder. I'm hoping it will give me better results than the 2X-S did. I'll have to wait until I get my slides back to know for sure.
Anyway, that assembly is extremely nose heavy and it makes it difficult to get the tripod head set where I want it. I started thinking about tripod ring mounts and raided my SO's lens bag. I discovered that the Canon tripod mount ring A is a great fit for the Pentax teleconverters. YMMV, etc. Tom Reese