The original baseplate is indeed aluminium, I had to make a base for my Tiny Tiger model 400 as it was also missing (they often are), one of the stalls at an engine show had an off-cut of stainless steel in the correct thickness at a price I couldn't refuse, aluminium would have been much easier to work with though.
I also made the little spacers that fit the screws between the base tank from paxolin rod.
The two screws are #10-32 with a slotted mushroom head, I had to order way too many to get the right size over here in the UK.
I never did found a source for the Acorn Pal-nut though, I believe the correct part is Palnut AC 1032 if you can find any.
David