I dug into my stash and found these ribbons I bought from one of my trips. Time to put them to use now. I sealed the ends with the same grosgrain ribbon I was going to use for the pen sleeves.
The velcro tapes I used came with quite strong adhesive backing.
I didn't have a standard length to the strap. I just made one to carry about 2 books and 5 assorted pens with me. So, with different requirements, the length of the strap could vary from for 1 book all the way to for a stack of 5 books, or more. Then again, if it should be any more, you need a bag.
In another attempt, I made straps instead of using ribbons for the strap.
So, this was the other strap I made. It seemed sturdier because of the double-layered fabric.