Decals over gaps

1: If you can lay the original panel over the gun bay, tack with pva/elmers, but make sure there is no pva on the surface.

Carefully lay decal over the cover & wing, (do not use solva set). allow to dry.

Then carefully slice the panel lines, peel off the the pva with a cocktail stick, & solvaset the decals afterwards if needed.

2: If you have a Tamiya kit, get the markings/decal layout sheet, copy it, lay decals in the correct position on the layout sheet, & carefully cut the decals.

Otherwise enlarge the markings sheet to the size of the model, & as #2.

Thanks for the tips! Especially using the marking sheet in actual size as a stencil! I'll give it a go. 

I have made decal windows for small scale airliners by just putting a single strip of clear decal over the strip of windows.  Since you will be cutting out the open areas (I assume) you will not be concerned about the sag of the decal in the opening.  I'd try it.  It would be nice if you copy and print a spare decal on decal paper first.