Gym bag. Has multiple pockets and can be "lined" with foam if desired.
Before I found an "eBay special" backpack rangebag at a great price, my DIY range bag was my gym bag and it did the job. I only bought the "eBay special" because I wanted to look cool, not because it actually added any real functionality.
If you were going to (or have) done a DIY range bag...what would you do?