I got the sandwich plate and the gauge from Merlin Motorsport
(www.merlinmotorsport.co.uk) at Castle Coombe.
The sandwich plate is known as a "Billet" sandwich plate, probably because it is machined from a solid piece of Aluminium.
You will also need an extension piece with a 3/4 x 16 UNF thread that screws onto the existing oil filter mounting thread. This holds the sandwich plate in place and also provides a new thread for the oil filter to screw onto.
The gauge is SPA Part No. DG209

I hope this is of some help