First, I placed the components on the base board to be sure that I had plenty of adjustment for the belt tension. Then spotted the holes for drilling. I used a mixture of blind nuts and carriage bolts because I had them on hand. The base plate was counterbored on the bottom to relieve the fasteners as shown. After notching the belt support and rounding the idler bracket, they were assembled to the base plate.
Large washers help to stiffen the joint.
After adding the motor, tighten the belt, and complete the belt support by fastening the shelf which the belt rides on. I used 3 drywall screws, but an alternative might be biscuits and glue. A residential light switch and enclosure was mounted. Make sure the switch is rated to handle the motor current. At 110V, a 15A switch is good up to about 1hp.