There are 6,200,686,124,225,191 free polyominoes that touch all four edges of an 8x8 grid (A268311), plus however many are 8x7, 7x7, 8x6, etc.

That's too many for my disk. How about sampling some at random? Generate a random 64-bit integer and check that it represents a connected polyomino and that it is the minimal representation after translation, reflection and rotation.  About 1 in 3000 integers should pass the tests, so you should have a large sample reasonably quickly.