Modification of 13x18 film holder for 5x7 film

Hello,

does anyone have a tip for modify 13x18 holder so that 5x7 film can be safely used?

I'm from Central Europe and this is the second time I've bought 5x7 holders, which are actually 13x18 I already have a good supply of them for my commonly used Fomapan 100 and 200.
But I'd like to occasionally use Fomapan 400, and it's only produced in 5x7 format.

Does anyone have a proven way to reversibly or irreversibly modify a 13x18 holder? Insert/glue some mask, or 3D print?

Thanks a lot.
Jan

I have some metal film sheath that was meant to use 5x7 sheet film inside 5x7 plate holders. I tried them inside 13x18 wooden plate holders, and they seem to fit with small room to spare.