I bought a Radio Carver a few years back and decided I should experiment with some of the many lettering and numbering templates I already had on hand.
I was most drawn to the brass ones that offer a pathetic 1/16th of an inch or less to track on, making for a LOT of opportunity to blow it at any stage of the project.
To solve the problem, I made an absurdly simple holder, which just holds the perimeter of the templates. Using Masonite, the brass templates are raised so there is about a 1/4" air gap under the templates. This gives about zero chance of blowing it using super thin templates of brass, plastic, Masonite or what have you.
From there, it was a simple matter to make negative patters for relief carvings too.