It has been mentioned before, but just to chime in, I have worked quite some time with the waste board milled flat and mounting the heated bed on top of that. Gave pretty good & consistent results. The added benefit is that it helps to insulate the bed which means it got to temperature a bit faster, but more importantly stayed a lot more stable.
Later I’ve taken it a step further and tried to create a quick-change solution that actually works pretty good: Idea for quick switch bed: feedback requested - #26 by brvdboss
(I should actually post an update on that as I did simplify the design a little bit to make it easier to create them)
The advantage is that these approaches are non destructive. I used water resistant MDF and it remains very stable despite the temperature changes it is submitted to. (disadvantages are the lost height & some extra weight)