I'm a little late in reading this, but did you ever find a nice pattern?
If not, I'm posting a picture of one I created.
I like it best of all that I found because I made my own modifications to make it easier to use and move around.
It's filled with sand, but ya have to make sure you pack it full so you get a good weight.
I didn't put a flat panel on it...it's only a bag of sand. That way it's easier to grab and move around....also, because without the flat panel, it can lay on anything...doesn't have to lay flat. It can conform to anything you put it on.
If you want the pattern, let me know and I'll post the pdf file for ya go grab.