OK, so here is the first modification I've made, and how to construct my version of the frame. You will need 10' of 1/2" PVC, cut up as follows:
5x 4 1/2" pieces to build front to back cross members
4x 4 1/4" pieces to build the horizontal base
4x 2 1/2" pieces for vertical risers to second set of front to back supports
2x 9 3/4" pieces for horizontal cross members/disc supports
4x 4 1/8" pieces for rear vertical supports and diagonal pieces.
8x 1 1/4" pieces to join PVC elbows directly together
10 1/2" PVC Tees
4 1/2" Side discharge elbows
2 1/2" 90 degree elbows
4 1/2" 45 degree elbows
After cutting the pipe in half to get it home, and making a few errors, I was left with less than a foot of pipe left over, so make sure you buy the whole 10' section (Lowes had 5' sections, and I almost bought one of those until I noticed that 10' was only 42 cents more).
After building the frame outside the bag I discovered that it would not fit into the bag, so if you follow my plan you'll need to build the base and the first set of risers and add them the bag, then add the second set of supports from front to back with a set of 1 1/4" supports. Then add the cross pieces with the disc supports. Finally add the last vertical pieces with the 45 degree elbows.