I use teflon sheet. It comes in thickness of .002 up to a half inch. I use .020 inch for head gaskets and .010 for everything else. It cuts easily with a scissors or razor knife. After many years of use I removed a head. The gasket came right off, wiped off easily with a paper towel and was reinstalled. .020 looks to be 10 bucks a square foot. 1 sq foot makes 4 complete sets for my demon v8.
The Farm Boy uses an o-ring as a cylinder head seal, works just fine. It's 1 3/16" od and fits in the groove that's made between the 1.200" od of the cylinder and the 1.340" id of the cylinder end of the wate rhopper. It just sets in the groove. A 1/16" cross section works but if your sizes are off a bit a 3/32 cross section should work fine.
I replied to your same question on my YouTube channel. I used a bs025 o-ring which worked fine. If you are in the UK and would like me to post one to you, just email me your address.