The big cut outs in the handguard more or less obviate the purpose of the handguard which is to protect the hands from excessively hot or cold barrels. I can well see someone getting the tips of their fingers onto the barrel through those holes. To protect from moisture a good coat of paint and or mineral jelly would do the job. Holes that let moisture out also let it in; they also weaken both pieces of wood.
I can't see any point in using a No.5 butt instead of a semi-pistol grip sporting stock with recoil pad as per the old PH sporters; it's intended to a be a "sporting carbine', not a pseudo-military piece. The No.5 buttplate is generally agreed to be for those who enjoy felt recoil.
Seems a pity to use up genuine No.5 flash hiders as there are so many buggered up No5s that could be restored with them. Likewise the other components like handguards, especially fluted ones.
No offence, it's an interesting piece as a one off, but if being made on a commercial basis, those would be my comments.