I am going to cut painter's plastic to serve as a groundsheet for my seedhouse sl 3. I haven't seen an "actual" groundsheet before, so I wasn't sure if anyone had advice on cutting it. Should the plastic be cut larger than the base of the tent? Should it contour to the base of the test? Does the groundsheet need to be staked down? Any advice is appreciated.

The groundsheet should be cut an inch or two smaller than the actual footprint of your tent to prevent water from getting under your groundsheet, pooling, and potentially wetting through.

No need to stake the groundsheet.

What David said, though I'd recommend cutting it a bit smaller yet--say 3 or 4 inches smaller than the perimeter of the tent. Those things have a habit of shifting a little, and sideways rain can get on a large one relatively easily.

Thanks for the quick and helpful replies!!