When I needed to solve this for my Mom, we took an inexpensive backwash brush and put a cotton sock on it. Put the cream or gel on the sock and rub it on your back. The sock can be easily taken off and washed once in a while. I bought a cheap foam one at the dollar store, but I think any would work - slightly curved handle was best. My husband has done the same with his back scratcher, but the surface for applying the gel is pretty small.
Sue

Actually there are such things! I use a topical called Outback Pain Relief and I know they used to sell something such as you describe. The product I use is a roll on form, it might also come in a pump bottle - look for a photo of a guy in an Australian type outback hat and then check and see if they sell an applicator. Also check Arthritis Foundation website, they might have something to use. P.S. I am not in any way related to the product I mention. I just found it several years ago, have tried many products and really like this. It is a bit oily but rubs in quickly and I don’t mind the odor of the tea tree oil in it.

@mayofeb2020, Makes me sad to hear each report of the many wildfires happening across our nation and the thousands of families displaced as a result. Glad yours was brought under control and that you are safe.

I recently recovered from yet another one of those dreadful from neck to ankle very painful times caused by an earlier back injury that continues to flare up and put me totally out of commission until slow healing occurs. I use a very soft bristle back brush from Bed Bath & Beyond to slather on the topical pain relievers for those impossible to reach back areas. I use a second one to apply body lotion mixed with a bit of Aloe Baby Gel or Avon's Skin So Soft during the exceptionally skin drying winter months. Following a shower they bring PURE relief and soak in quickly. Hope your PT Monday speeds your healing!