Sheeting firedoor with ply

I'm making a "strong" but cosmetically acceptable (I hope) external door out of a fire door blank and a sheet of mahogany faced 18mm ply.
I intend to cut out 4 rectangular holes in the ply so when it goes onto the firedoor blank, the whole thing looks like a decent external panelled door once I put some mouldings round the inside edges / lips of the "panels" and surround the outside edges of the door with 1/2" thick mahogany (framing/sealing it).
It's a strength/cost/cosmetic compromise! I got all of the wood for 60.
Obviously I'll seal/protect it all heavily and use good woodglue /clamps to mate the ply & doorblank surfaces.
My main question is, in addition to the glue, will I also have to pin or screw the ply to the door to prevent shrinkage/warping at different rates?
I'm happy to screw it, but how will I best cover the screw holes up - will plugs be obvious? I intend to stain / varnish.
Thanks in advance Paul
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