Cutting rectangular hole in bulkhead

[doug748]

.....
Please arrange some other kind of carrier for your instrument! Use a spare washboard or bracket that hinges out into the companionway, but don't damage your perfectly good GRP. You'll regret it.

If he does not regret it, the next owner probably will.

As you hint, it is sometimes possible to use a box, possibly teak or whatever, which can be fixed using existing screw holes.
I have fitted a whole fascia across an existing narrow bookshelf, it makes a reasonable stab at looking original and can be removed, reworked or scrapped at any time.