C Extension Pulleys
I now make two pulleys especially for extension builders. They use an Igus™ plastic sleeve bearing which has a 350 lb. static load rating. The large pulley is 22mm O.D. x 5mm width (~7/8"x3/16") The smaller pulley is 1/2" (12.7mm) O.D. and is intended as a secondary for multi-pulley designs. The included shaft is 1/4 inch (6.35mm). Inch drills are available from me for metric countries.
A feature of the small pulley is that it can be installed from the side. You can bore a 1/2" or 13mm hole partway into the extension body and then a 1/4" (6.3mm) hole through to the other side, and cap it with a brass spacer. If you want to do this make sure to ask for the spacer and an untrimmed sleeve.
Previously I offered a 1"x3/16" sealed bearing pulley. This is still available, but I recommend the new pulleys which are cheaper and better suited to the application.
All pulleys work for strings up to 1/8" diameter. This includes almost all Long Es except Oliv, Labella, or most Nylon tape wound strings. If you want to use a thicker string, I can adjust the groove radius to your spec. I can even make up a custom pulley, if you want different dimensions. Custom pulleys are available with either the Igus™ sleeve bearing or a sealed ball bearing.