My system.
Install time: 45 minutes
cost: $3 framing angle; the rest commonly available for little or nothing.
Durability: good. Been using this one for a year. Haven't had it come off the bike either.

Back is a pair of framing angles from Ace (bent with a vise grip) and bungee held with zip ties


Add something so you can lash the shoulder straps:


Pick up some left-over Fastsigns and run some screws through the framing angles.