Although not a tether or leash, I read in an old Field & Stream mag.
In the projects section of using PVC conduit lashed under the thwarts along the gunwales of a canoe.
For use as sheath's to store your rods in...Worked very well on my old town discovery 16, Allowed the bro in-law and I to each keep three rods rigged and ready at all times...Three 1 to 11/4 inch tubes along the starboard for me and three on the port side for him..he took the bow seat.. and I the stern...there in was the prob..as the steering of a tandem is Supposed to happen at the stern....
Anyway... It was a sweet setup...still kicking myself for selling that boat!!!!
Wish ya great fishing,Bill