Before I was able to buy new fabrics, I used upcycled fabrics ALL the time. Since I was going back and forth with each child, I would inevidably give them all away, but they worked fine through most of one child's diapering time. They weren't so much of an investment and so the sacrifice was just time. I even used upcycled fleece to make all in ones.
So, this is what I did, used the best fabrics for the outer layers. It looks like all the flannel receiving blankets would work great. Then, cut up and use all the not so great fabrics for the hidden soaker pad. So, you need two whole bodies for the outer, and how many ever layers you want for the hidden middle part. Sort of like the hidden layer in a PF, but not from front to back, a few inches in.
Make the tabs longer in the back then the front. Zigzag the elastic through the full top layer at the back and legs, or you can do casings. Depends on your preference. If the outer layers are really thin, then you may want to do a 3rd full layer that may not be as pretty as the outer two.
If you are going to turn and topstitch, be sure to make your pattern with a seam allowance. You didn't mention a serger, but they can be really quick to get diapers sewn together.
Since you have all those prefolds, you can use that one prefitted tutorial that shows cutting out the sides just a few inches from the back to the front. Then, take those and get some extra tabs from those strips and attach to the tabs on the PF still attached. Add elastic to that in the back and sides. Then, cover both sides with your flannel blankets. You may or may not want the rest of the cut outs for the middle soaker. Depends on the thickness. If you T&T, then you will want to cut your flannel an inch bigger all the way around your PF, and find a way to attach that PF to one of your flannel layers. Put right sides together, stitch leaving a hole, and turn right side out. Slip stitch the hole closed, and voila! A fitted diaper.
You can also use those other supplies for lay in soaker pads for nighttime, or if you have a heavy soaker. Either do a tight zigzag around, or T&T, or serge.
I have never had a problem with "inferior" upcycled fabrics for diapers that are for only one baby, maybe two.