Yes, I used to get a nice piece of oak (or other hardwood) dowel and drill a couple
of 1/4" diameter holes through it about 1" apart and about 1/2" off center.
With the 2 holes, you don't need the cleat (which usually jams the line and is very hard to get loose after you finish downhauling. Just run the line in one hole from the front, then back forward through the other hole and cross the line back over the outside of the tube/dowel and tuck it under the loop between the 2 holes on the back side and you have a great little downhaul tool.
Then if you use only your legs (not your back) to do the pulling, you might not give yourself any long term injury or back problem.
But once you've used the Rig Winch or other "crank type" downhauling tool you will quickly see that all the pressure that you used to absorb in your legs, and lower back is now completely handled by the tool.
A fairly well known WS personality here in the US (who owns a big shop with rentals, lots of rigs, and is his only source of income) was having some serious lower back problems.
When his diagnostic X-Rays came back, the doctor asked him what he did for a living as his spine had lots of damage similar to a "hod carrier" (hod carrier - a laborer who carries supplies (concrete mortar) to masons or bricklayers) up onto the top of brick buildings under construction.
Well, it turns out that the damage to this fellows spine was directly attribtable to injuries from downhauling lots of sails.
So, you can continue to use your back and legs, and risk temporary or permanent damage to your lower back, or simply get a downhaul tool that eliminates this risk altogther.
Hope this helps,