Cut a 3/8" piece of plywood this diameter and drill a series of holes around the board's edge. Sand carefully. This will be laced around the tube with 1/8" nylon cord.

Construct a pine or plywood box as shown and attach to the circular board. Paint or varnish as desired. I cut a notch to hold my rod as noted in the drawing.

A nylon line with a clip at each end will hold the tube box to your waders at a distance that is comfortable to your needs.

You can now wade in comfort and safety, if you should happen to step in a deep hole, your tube box will immediately come to your aid. No effort is required to tow the tube box and moving upstream it will stay behind you until needed.