Re: Cheap shooting bags.....$$
I use a commercial bag up front and a shot bag filled with sand in the back tied shot with braided copper wire(ground-strap). I've also got one filled with the beads from on old commercial bag I had once.
I do like my old way better, though. My dad's got that set-up, as when I moved out I left it there. I took a 1x12 plank and cut a bunch of squares out of it, then screwed them together 'till they are 8 or 12 layers deep. You then toss a shot bag or three on the front and one in back, and you've got a heck of a rest.
A side bonus is you can use your rifle support to help boost your truck's transmission into place, been there, done that on the last truck.