Ek, it is a lot of work, but very possible to make a backing from an ash tool handle, like a shovel handle, or an ax or pick handle that is hickory. You have to look at a lot of them, maybe go to two stores, but when you find one with straight grain, both rings and radial grain, it works great.
If you get an ax handle, saw it in half along the widest part the whole length, then plane or saw off the excess from each half.
If you get a shovel handle or rake handle, look for one that has thick growth rings. and chase off the rings that stand high one after the other, until the part that will be the back is wide enough and most of the roundness is gone except on the edges. So, it is more like a fat rectangle cross section, not a circle. Then saw that into two halves and work the sawn side flat and even thickness.