I read about the MDPE and sounds like it is strong stuff. I would be concerned about getting it level and no voids like
@GBBUDD stated. I would also be concerned about pockets, underground shifting due to time, ground water seepage, etc. Once it is in, it is in and that would be hard to correct. A liner just goes w/the flow and settles w/all the shifts. I know some on here have used solid items to hold fish in and seems as tho they all somehow fall out of level eventually. I think the larger tho, like swimming pool size must work as lots of people have swimming pools that don't seem to go out of level that I know of anyways. Maybe cuz they are so large and heavy? Not sure how heavy a pool would need to be to not shift at all.