If you do a search for antenna mod you will see how I made my internal antenna coils to replace the non scale external antennas on the HL tanks. The thread shows how and where I mounted my internal antenna mod.
here's the link http://www.rcuniverse.com/forum/m_71...tm.htm#7195692
As for cutting sheet styrene you can score and then snap or use a razor saw(zona saw). Since no actual stress will be applied to the mount .030-.040" styrene can be used or just hot glue the antenna coil to the side wall of the tank