re: Anyone ever set a Resin shed? Not sure what foundation to use.

Not 100% sure what a resin shed is so maybe my answer won’t work but for mine (16x12 I think) I just poured concrete runners. Think I made them about 10” wide x length of my shed x 8” or so. So for a shed the size of yours that may be less than 10-15 bags especially if you throw a bunch of free trash rocks (old busted up concrete I picked up from here and there).