Thornton-Pickard shutter question

While checking my TP Imperial for light leaks (only one in the bellows), I noticed a fair amount of light leaking around the TP roller blind shutter on one side. Investigation shows that the shutter box is screwed directly to what my brain calls a lens board, and the tension wheel on one side is preventing it from laying flat.

What should normally be between the shutter box and the lens board, since this looks like a pretty standard design? I'm thinking I need some kind of felt or fiber-board spacer to fit between them and seal out light, as well as leveling out the shutter box (I suspect this is why the lens board is frustratingly difficult to slide out).

Many times these shutters are screwed onto the lensboard, and some are indeed made for that purpose. However many have been made to put on the sunhood of the lens - your shutter might be one of those if it has a setscrew by which it can be fastened on the lens hood or lens ring.
The roller blind shutters that were intended for mounting directly on the lensboard did not have parts sticking out like yours.

In your case you could put a piece of wood as big as the shutter between the shutter and the lensboard.