Simple 3D printable back scratcher head, for attachment to 10mm diameter dowel / tube / rod of appropriate length - about 30 to 40cm works well depending on the height of your person / length of your arms / joint flexibility.
UPDATE: 13/02/2018 - extended the solid receptacle section by 10mm in an effort to make the 'fingers' less susceptible to snapping off. Also added 'scratch' embossed text, just for fun (but added an identical version minus text, if makers don't like it!).
Print with the receptacle flat on the bed (standing upright). No supports required, but you may have to reduce print speed while priting the 'fingers' as they could overheat if printed too quickly, particularly the overhang section towards the top.
You may like to print on a raft or perhaps with 5-6 brim lines to give the base a bit more stability, though I've printed the item a few times without either and it has been fine.