I'm  making something similar to this


which holds this



So I need to take a mould off the back of the unit above to get the profile.  It needs to be a snug fit in the holder and I'm wondering what thickness of wax sheet would be best to get the profile correct, 2 mil or 5?

Also when the wax sheet is on, I've seen that some put packing tape over it to prevent sticking, is that a definite method because surely it means sanding work on the mould to remove any lines caused by the tape?