The 5 old stock winches have a bar welded that keeps the strap close enough to the load that the curtain can close easily.
The problem with them is that they cant hold a strap all the time because the bar pinches the strap. I have 30' straps. Well had. I had to cut down three straps to make them fit under the welded bar. PITA.
I will probably take off three of the movable and put more bolt ons under the rail. Leave the old ones where the tires are. The sliders sit on the side. The bolt ons are under the rail.
I use about 20 8" tarp bungies to hold the curtain where straps are missing or not good. I'm thinking of just replacing all the 50 straps with bungies. It just seems quicker and bungies don't work loose.
Are you rolling all your strap up on the winches? I always roll up the excess leaving about a two foot tail, tuck the extra strap under the load (or lay it on the deck), feed the tail through the winch making sure it goes all the way through, and tighten.
I find the straps don't work loose, get tighter, and it looks neater compared to rolling the whole strap on the winch.