I liked Zormis's telescoping back scratcher [on thingiverse] but even at 100% infill after a couple uses the small [threaded] sleeve would break from being too thin. This lead me to preferring the bigger more sturdy telescopic back scatcher mistertech [thingiverse] made. However, I preferred the sharp edges of the large brush head from Zormis's version. I combined that head with the threads from mistertech's and now I have my perfect back scratcher.

Can be printed flat on back, no support. I did it at 100% infill to avoid weakness. I printed both the handle and head at .3 layer height.

There is a bit of resistance when threading them together, but you can lube it up with some WD40 and it should help out a bit.