...So scratchbuilding one is a great solution indeed...
Well bother; only when looking at the thread on my PC did I realize that you
did use a toy as a basis, although it's not perfectly clear to me which one.
Apologies; suits me right for browsing the forum on my phone and then not bothering looking over a complete thread...

...Can I suggest you maybe put some tin foil on the inside of your engine? That'll cut out the light bleed through the plastic card. Even painted, it'll still show through.
I can attest to that.
An even better solution is aluminum tape, used to seal off air ducts (as opposed to duct tape; go figure

). It's actual metal, so absolutely lightproof, and it being tape, you won't have to bother with glue to put it in, the foil wrinkling or tearing, etc.
