Disc bags are either tiny and cheap or crazy expensive. If you want a back pack style bag you are going to easily pay $150 plus. I came across this picture of a bag used by redditor martial_arts. After poking around on bag-related posts on r/discgolf I came across this post in DG Course Review that describes how to hack a cheap backpack from Target. I only made one small modification to the original design to add an additional storage area in the bag. All things considered this version cost me $20! If you don't have any pvc pipes and fittings the cost will go up by about $5.
Step 1: Find a Suitable Backpack
I used the "Jar Top" backpack made by Embark. These are available at Target and on their website. I picked up mine on sale for $20. They also have an "elite" version for $10 more with thicker/padded fabric. I opted for cheap and light. If I need to fix some stitching or add some duct tape later it will just make me like the bag better.
If you opt for a different bag you are really looking for two characteristics. The most obvious is that you want the storage compartment to be wide and deep enough to accommodate your discs. The other is that you want a bag that doesn't have zippers that split the bag down the sides. Remember that the whole point is to be able to walk around with it open and not be afraid that all your discs are going to fall out!
Step 2: Build the Frame
I used 3/4 inch pvc that I had on hand. It should be fairly obvious from the picture what goes where. I cut the horizontal pieces 25 cm and the vertical pieces 18 cm. The shorter sections of pvc that I used across the top and to connect the feet were 10 cm. The origional design didn't have the feet at the bottom. I wanted my discs to ride a little higher in the bag and this also allows for storage under the discs (extra shirt/towel, a couple brews, a blue tooth speaker, etc).
Step 3: Pack Your Bag!
The frame should fit snuggly to maximize the number of discs you can carry. If you build to my dimensions the top flap should just barely close without having to stress the seams or zipper.
The top and front flap can be tucked in so that you have a couple easy access slots for your putters. There is a lined pocket on the top for your valuables and two bottle holders that snuggle accommodate the big fat nalgene bottles. Don't forget a couple golf balls to get discs down from trees and some sharpies to autograph your aces!