I ripped out several sheets of mdf that's around 3'x8' in my attic. Is this strong enough to make a seat and/or foundation for 777 mip?

All depends on structure ... I could make a seat out of printer-paper if it would be asked ... remember how you could stand on an egg if you only put the pressure on the right side of the egg.

I've used MDF in my 777 cockpit and it is absolutely strong enough. Be sure to use at least 3/4" for the main structural stuff. Steve