Hi rcseisland,
They both sell plywood yes. I would use indoor plywood that is at least 1/2" thick, untreated, and sanded on at least one side. It may be worthwhile testing your mattress on the floor before buying the plywood to confirm that the foundation is the issue.
It’s certainly possible that your mattress is too soft for you and that could also account for your symptoms so I would trust your own perceptions and personal experience and “best judgement” over my guesswork.
Latex isn’t temperature sensitive like memory foam so the weather wouldn’t be affecting the firmness of the latex.
Hopefully the firmer support from the plywood will solve the issue but if you do decide to try a topper then post #2 here and the topper guidelines it links to includes a link to the better topper sources I’m aware of and a link to the topper sources I’m aware of that have a return or exchange policy as well.
Phoenix