I should also add that my original flat plates were mild steel and even at 3/4" thick with a 1" x 2-1/2" backer plate these plates got scarred up and would even bend. Mild steel might work for a 24 ton press, but for a 60 ton you need more resistance. I'm in the process of making new plates and using 4140 that I'm hardening and tempering. Some of the tooling I place on these plates are also 4140 and others are mild steel that I have hardened.

In my humble opinion exposed welds can detract from a forged piece - even if they are very nice or cleaned up. Try notching or drilling the bowl itself and plug welding from the other side if you don't like rivets or mortise and tenon. If done well it would be plenty strong. Assembly of small forged pieces has always taken a lot of creativity and design foresight for me. To bad we can't attach things with magic:)