I dislike sticking a lever into some dark space to tension a belt. It is easy to lever against something fragile, or find you've lost the tension when everything is bolted up tight. I had never heard of a belt-jack before reading this thread, but it is an elegant idea and appealed strongly. My junk box yielded two scrap turnbuckles, each missing the left-hand threaded male part. I reamed the left-hand female thread out of one (red ring) and voila! It works like a charm. The pics tell the story. The little pieces of clear hose are enough to stop it slipping on the pulleys.
BTW: This discussion came a the right time. The belt in the pic was worn and making black dust. I have used my little "jack" to change belts on both engines. Can I hope that the black dusting will be reduced, or stop altogether?
Sorry RWidman, no patent infringement intended!