If you are keen on using a "paint", have a look at this stuff, more of a plastic coating than paint.
Conventional paint is a bad idea.
You do not say how your circuit is constructed, but if it is on a piece of circuit board or strip board you could fix it to the inside of the tin using double sided "sticky fixers".
These usually have enough thickness to hold the board in place and provide clearance between the circuit and the surface of the tin, if not use two sticky fixers, one on top of the other.
I have used this technique myself a few times when I needed a quick and dirty method of installing a modification into something.
JimB