There is a building product made to seal wood that has rotted to avoid having to replace it, for example in window frames. Here is a link:

http://www.screwfix.com/p/ronseal-we...ar-500ml/63540

MDF soaks it up like a camel at a waterhole, and it's surface gets a lot harder. It dries quickly. I would think if you bought mdf and let it stabilise, then treated your components with this stuff, and even with West System epoxy for good measure after assembly, it would all be a lot less sensitive to the humidity level.