In my opinion, this is a tough area to get sealed and expect it to stay sealed over time. A steel dome is easy - weld it. Larger triangulated space frame type aluminum domes are not well known to stay liquid tight over time, much less pressure tight. The edge is particularly problematic - dissimilar materials, polygon shape meeting circular tank, etc. I'll bet it has been done and probably can be done ( for a price ). Vendors are your best bet, find ones with experience and history, not just pretty sales talk.