You can build a belt tensioner out of 1 ½” by ¼” flat steel or use an old clutch assembly. If you use flat steel cut 2 pieces about 6” long or according to what space you are going to have. Bend a joggle in each piece about ½” high as in you can do this in a vice with a torch and a hammer or if you know someone with a press brake that can bend it that’s even better. After bending drill a hole the size of the pin or bolt the tensioner will turn on thru both pieces. Drill a hole in one piece to fit your pulley or sprocket if you are using chain. Drill a hole in the other to fit your J-bolt. Put a bolt thru the 2 pieces and make sure both ends a bent up as in make them at a right angle. And weld them together. Add your pulley, mount it to the frame put your J-bolt in the other end and see where you can weld your piece of angle so it doesn’t get in the way of the belt. Pull the pulley up tight against the belt hand tight the nut on the J-bolt then tighten it with a socket or wrench until the belt is as tight as possible and will still turn easy. It is possible to get it to tight.
