Can I have some vinyl? thanks
Also, what sewing machine do you have, industrial?
To the rest giving design advice, yes its quite difficult. Remembering that it sits on a human being that will wear it for hours while even exercising is a key design element, comfort.
To the poster, I would not expect to find a pattern that is as good of quality as these professionally sold items, you would have to research their designs and borrow elements from them. I don't know of another way perhaps others can say. . .
Also, to those who want an extra pockets or a u-lock holder, are you not interested in just making those customizations? or would you want to hire someone to design and build it for you? Needle and thread and some scavenged nylon straps are sufficient but require a bit of skill. In some cases an ordinary sewing machine is up to the task, but not for long. Lets put some pockets or stuff on your bag, it should not cost a ton for me or someone else to do it.
Durability is one other design element I've seen amateur bag makers miss. Being able to design the bag to last is a consideration of these professional offerings. Forces add up and its not always the stitch that give out, sometimes its the vinyl or the structural elements.
I'm a fan of my sealline bag. It's actually pretty ginormous and a little too big normally but that comes in handy at times. It has this handy closure system where a metal hook hooks into rubber ladders. It takes a short while to get used to, but it lets you open and close the bag with one hand and without looking. It doesn't have exterior pockets which is a little annoying but it does have attachment points for various pouches that you can add.