I made my easel base using two plies of corrugated plastic (like used in yard signs), glued together.  I glued leather straps on each corner to mount the four rings, then used small carabiner clips to attach the harness to the rings.  For the harness I used light nylon camp rope.  I found the adhesive magnet material too weak, so I used some high strength 1/4" magnets that I mounted and glued into the base.  I used Gorilla glue; it's waterproof and quick-setting.  This easel works great and is very light-weight.  Here's a sketch I shared with our DFW Urban Sketchers.