As long as it is looks somewhat square, you can open it, putt stuff in it and carry it around, you should be fine.
But you really want to make one yourself? They are pretty damn cheap, around 10-20 pounds/euros.
Nah don't want to make one, me and my other unit researcher are getting slightly pissed off because everyone in the unit now has different bags, we want to be somewhat uniform, last year we changed shako suppliers because looking at the unit there would be at least 4 different manufacturers of shako on parade!
The bags we currently have couldn't hold a whole day's rations let alone the 3 days that were usually taken from stores!
There's always going to be differences among members. It happens for a number of reasons, some people specifically only buy the cheapest things they can find, others want their items made a very specific way. My unit for example has gibernes from at least 3 different suppliers, heck some of our members even have English style cartridge pouches while we should have French styled ones. Most of my unit get their gear and supplies from Coopers Choice, a few from McFarthingbowl's, I personally do it completely different and order everything from Shakos GBL. And trust me, there is certainly some small differences in the way these items all look. While I agree there should be a certain level of uniformity, there's always going to be individual differences. But as long as they remain small (different bags seems acceptable to me, just as long as the basics of the uniform and gear are uniform). And whether you like it or not, it's gonna happen.
But that's just my opinion on the matter.