Get yourself an old boom, cut a section out around 30cm.
Drill a decent size hole right through just off centre or big enough to easily slide your downhaul rope through.
get a small cleat (Clamcleat) from ship chandlers or windsurfing shop, drill 2 holes approx 8-10cm from hole for rope using the cleats holes as a guide. Pop rivet cleat onto section of boom.
When downhauling thread your line through hole and then cleat off. You can throw in a couple of half hitches before you cleat off if needed for added security of punching yourself in the head if rope slips

.
Most important, use your leg muscles and push the rig away from you thus saving your back.
Or just buy a rig winch