I've great success with homasote. I still have an unfinished module in the basement that had seen attics, garages, etc and seems unscathed. I simply glue it to the plywood with Elmers wood glue.
Damn yeah, it is so 1970s (hell that's where I dismantled my last non-modular layout).

But here the fudge can I find this Sintra stuff for a look see?