Been thinking of building one also. I've be using a Home Depot canvas tool bag, and like Ray-Vigo said, the more you go shooting, your bag gets smaller. It's like when I used to backpack. At first I carried a heavy pack with everything I could possibly need. Then I started unpacking after a trip with a question for every item coming out of the pack. "I never took this out the entire time, and it's not an emergency item." "in an emergency, I better keep this." And, "Could have done without."
My last backpacking trips I bet I carried less than half of what I started with.