[Public-List] One for the builders

[GregR via Public-List]

This question isn't specific to our boat but I think the answers will be of general interest so I'm asking it here.
One of my neighbors has a Uniflight power boat that needs a new cockpit sole including support beams. He wants to do a decent job but I haven't convinced him it's worth the money to use BS1088 ply.
What are some decent materials for the support beams? What is the second choice for plywood?

Regards,

Greg