I picked up a section of Sonotube yesterday but had to cut it in half at the warehouse since I couldn't handle a 12' piece of 20" pipe in my truck. So I brought a saw with me. I also picked up a 4' x 8' sheet of styrene at a local plastics distributor. Since it was the bottom of the stack and very dirty I got it for almost nothing.
To trim one of the sonotube pieces to an estimated size, a laser level was used. After drawing a line, the tube cut easily with a jig saw. The one drawback of the sonotube is the stickiness of the inside of the tube. Kind of like a wax inside. Since Sonotube is a concrete form, it makes sense to have some sort of mold release on the inside of the tube. I need to think of some sort of liner for the inside if it's to be a wearable robot.