I would put your bench in place and see if it moves much. I have some large steps in the cockpit of my boat and they used to be attached to the transom with long hooks. I left them unhooked one time and they really did not move much so I stopped hooking them in place for a while and they stayed put pretty well. Only would need them fastened in a big seaway. Some brackets like these would work nicely but you might have to make a tapered wood base plate to adapt them to the curved transom. Sea-Dog Ladder Locks - Boarding Ladders - Deck & Hull Hardware - Downwind Marine Nice looking bench. I like the two piece idea to make them easier to move.