Can anyone give me some suggestions on making a bulkhead basement door?

I have a 150 year old house so a pre-fab door just won't look right. The opening will fit a 4X8 sheet of plywood to use as a door. However I'm a little concerned about the weight of treated plywood. Does anyone have any suggestions or alternatives to treated plywood? Will regular plywood work if I'm careful to seal it up with a Thompson's or similar product?