For internal routes such as control cavities you may find it easier to use a guide bush. Then you can use thinner templates such as 6mm MDF and get a longer plunge. It does mean calculating an offset (the guide will be a larger diameter than your bit) but you don't need a bearing on your cutter.
Edit: And it's safest to do a test route on scrap wood first to check you've got the offset right (yep, I've had to re-make a couple of templates, but better than getting it wrong on the guitar!)