Roger,One of the most common pieces of 'flat' are the marble pieces sold for window sills, and you can use them for backing sandpaper while 'decking' your jug of cylinder head.
That is an interesting suggestion. To add slightly to that… if there is a marble installer local, I bet that you could stop by and ask permission to get a piece of scrap marble from their refuse pile. There is an installer close to us and behind their shop is a small hill of marble remnants – plenty of which are large enough for performing the type of head work required. My wife and I rummaged through our local marble supplier’s pile (with their permission) to find a specific color combination that we were looking for. This approach could work wonderfully for getting a stable, flat surface to smooth the bottom of the head - I love free!!!