So, i had been entertaining the idea of a bit of wood across the bootspace with a fork mount bolted to it so i can stand my car up in the boot.

Just this minute though, i thought to myself,
“Can i bolt the fork mount to the plastic cover for the soare wheel?”

My idea is to drill two holes in that, and in the corresponding place in the boot carpet, and fit the bike mount. The bike would also be held using straps to the cargo eyes.

Any reason, either regarding stability, fragility of the plastic etc that make this a bad idea?

What kind of plastic? How thick?

You could reinforce the mount with a bit of ply either side to make a sandwich with your cover in the middle, I’d imagine that would make it more than sturdy enough. The key is to get a few (machine) screws through to spread any loading on the plastic as your bike moves, preferably as large a diameter as reasonably practicable (so not super thin but m12 coach bolts would be daft).

Not a plastic expert, ABS at a guess.

Its a vw passat, looks like this

Have you thought about something that’ll grip the carpet, like the ‘hook’ half of velcro?
Enough of that on underside of your piece of wood that your fork mount is screwed/bolted to should hold it without any need for drilling any holes?

Of course i thought of that, but i am trying to avoid buying something if i dont need it.

Am not asking about wood.