Any coil for the Prizm line will work. There are on the Whites website at least 3. The 9 inch spider, the 9.5 inch Prizm 950 and the Prizm mini DD coil 4x6. The 4x6 is a killer in the high trash areas.
I found some silver coins in my local park using the 4X6DD shooter coil on my old Prizm 6T. The ability of that little coil to get between junk signals and separate them is a big advantage. It also has excellent depth capabilities for a small coil. When I bought my DFX, I made sure that the 4X6 was in my arsenal. I would highly recommend it for your Coinmaster GT.