Muskrat, do you have some base gaskets to compare, A65 - A10. You are welcome to come visit anytime.
The way I made my A65 cyl was to get an old std cyl and knock every second fin off it, that allowed me to easily fill between the fins and thicken it to the shape I wanted with body filler, it had to be built up everywhere a bit because when its cast there is a little shrinkage. Then I took casts from it and from them made sections in F/glass resin, those sections I then attached to boards and painted with a type of paint the treated sand does not stick to. I made it pretty simple with only 3 patterns; front, back and underneath. There is no air passage between the cyls because in alloy its unnecessary, stronger and simpler to make.
So to make patterns this way, you would need an old A10 cyl that is no good, even if it had a broken flange that could be stuck back together would be ok. It would be a bit harder to do keeping all the fins but making something from scratch would be more difficult I expect.