Clamp-on side handles used to be available, but when I looked for one a few years ago they appeared to have gone off the market. They used to be included with some larger size electric drills.
My particular need was for new handles for a scythe that I had bought at a garage sale. I took a 9 inch long bit of close grain 2×4 spruce, turned a 1-1/4 inch diameter handle 4-1/2 inches long at one end, drilled a hole through the wood slightly larger diameter than my scythe handle (can't remember how, probably drilled a series of small holes and then filed the inside smooth), sawed down to the hole from the end of the wood so it could be clamped, and added 1/4 inch diameter clamp bolts. A hardwood would probably be better than my spruce, but the spruce was handy. Contact me if you would like a photo.