Carl.
the "which" question relates to the distance from the reducer or flattener to the chip. On most it is about 55mm, and fitting an OAG into the mix causes this 55mm to bloom to something longer, which then introduces issues with the incorrect distance. Given that the Hotech is probably 55mm, and that you are using a DSLR, I think this could be the issue here. If it is, then the thinnest you can find, the TS 9mm sucker is the one that springs to mind, but I've read mixed reviews on it.
In checking the Hotech specs, I see it lists "19mm" as a back focus specification. If this means 19mm from the rear of the flattener to the chip, then you are stuck. If it means 19mm from the flattener to the T ring then the TS 9mm job could work.
I know others (Houghy I recall) have had them and used them, so check with them.
Good luck.
Gary
Look here: http://www.teleskop-express.de/shop/...-only-31m.html - I'm using one of these including their 2.5" FF (I still have the HoTech I started with) and it's very versatile; allowing me to use them on both my SW BD 120mm, SW Equinox 80mm ED as well as the TS Photoline 80mm Triplet - can't complain when you can use the same kit for all three OTAs!