In my quest for the perfect warp, I've made some big adjustments (switching from front-to-back, to back-to-front) and some little ones. Some things I've changed, not because they were actually problems but, because they were awkward for me or less efficient. A means of separating the warp on the back beam has been one of the little non-problems for me.
My weaving teacher used rolls of corrugated cardboard, the kind elementary school teachers use for bulletin boards. I got used to using these in class and looked for some for myself. I even visited an area teacher supply store, but when they didn't carry it I didn't know where else to look.
Next I tried cutting up paper bags, but in trying to achieve a solidly wound warp I eventaully abandoned these. With paper bags I still had some mushy spots along the warp beam, and more than once I've wound them on crookedly (which does create a problem).
Peggy Osterkamp says that if one's warp is very tight and smooth across the beam with no gaps in the warp groups, then separators aren't necessary. I'm still working on this one.
Sticks were the next option. These work okay too, unless the warp is very long. In this case I run out of sticks, plus the warp beam ends up very fat, which I don't reckon is actually a problem.
So when I was getting to toss an odd mini-blinds into the give-away pile......
........... it occured to me that I could use the slats as separators.
I still use sticks to separate the warp until the apron cords are wound on, and then switch to the slats. They are firm, lightweight, add very little bulk to the back beam, and nest (somewhat) for storage. Plus they end up being cheaper (think thrift store or el-cheapo discount store) to purchase than sticks from the hobby store.
You can also see from the photo that I am doing a better job of winding on without warp gaps. It was especially bad up until last May, when I started consciously trying to improve on this. I can't exactly tell you why it's better, but if I ever get it figured out I'll let you know.