48" loop runners at baileys are $8.95. At 55 cents per foot, you will pay $4.40 in materials. So there is a $4.55 per strap premium for getting it made for you [plus shipping!].
There is another consideration: all the loops from Baileys come in a different color for each length, making it easy to grab what you need and to do a quick visual inventory of what you have with you. Don't underestimate how important it is to easily grab the right loop when you want it.
When I am up in the tree, I clip one carabiner to each loop, and then carry all of them clipped together on my figure-8 or daisy chained down from that. So all the 3' loops are in one string, ready to use with a carabiner on them, and each of the other lengths is too.
The figure-8 gets a little crowded, but it does keep everything organized, and you can unclip it and hang the whole mess from 2' loop tied onto a branch or whatever. Plus, the figure-8 is always there if I really needed it in a pinch, too.
If I was going to make my own loops, I would buy some 12-strand rope and splice up some double eye slings. Smoother, stronger, and more reliable than the loops with knots, and the hollow 12-strand rope is real easy to splice. A short sling with two eyes would also be more versatile than a simple closed loop. You could always double it up to make it shorter and stronger, or run it single strength for greater reach.