I assume that as a "poor college student" price is the main reason you are making your own pack. By the time you are done, I do not think you will save much over searching the internet for packs on sale. External frame packs are really cheap and although when you get some money, you may prefer an internal frame pack, you can do a lot of backpacking with a $50 or less external frame pack. I picked up a nearly new Kelty external frame pack at the REI "Garage Sale" for $20. You are young and strong - a bit heavier pack may not even be an issue for you right now. In my "poor" days, I had heavy but adequate gear and just put up with it, got out and did a lot of backpacking. Then when I got a job, I steadily replaced the heavy stuff with lighter stuff.

On the other hand, if you really enjoy the idea of designing a pack, go for it! When designing and making your own stuff, it is a lot of trail and error before you land on a good design. I do not think you end of saving much money in the process, but if you enjoy it, it is a lot of fun.