Thanks for all of your replies! The milk crate option sounds like a very simple solution. I may need to double up. I can just strap them together and I think that I'll get the right height. I need to find the real milk crates, not the cheap ones that they sell at office supply stores. Maybe I'll hang around a Farm Store or 7-11 tonight...

I'm going to look into the suggestion that 375Brno made as well. I need to scope out the pillars and posts that they have but I think it may be impossible because it may take out a few shooting positions at the same time.

What type of "rest" should I use for the portion where the arm rests (on top of the crates)? Some type of firm cushion?