Archie,
I just picked up a canvas bag from Wal-Mart. It’s larger in size, but closely resembles, the rectangular bag we used to keep our pencils n’ stuff in when we were in grade school. It’s heavy duty canvas material, and has a zipper that runs along the top lengthwise. I simply filled it about 1/3 – 1/2 full with pea gravel, zipped it up, and ran some duct tape along the zipper edge for extra support. It doesn’t look anything like the one Damian uses in the videos, but for the purpose of something to rest my brick on that has a little give, it works beautifully.
Of course, your sandbag sounds like it will do just fine. In fact, I started out by looking for some type of sand or beanbag myself. When I couldn’t really find anything that I thought would hold up to the punishment I intend on dishing out, I figured, what the heck, I’ll build it myself. Like I said… it’s damn simple, and it certainly ain’t pretty, but it works! 
