A gym bag, you remember them, 18 to 24 inches long,,Takes some doing, but heres what I did,, I sewed a flap with velcro on one end, it wraps & straps to the "handle" acroiss the seat. I sewed "D" rings on it & worked parachute cord under the seat, to tie it down. I used an upholstery machine & sewed a piece of panelling to the bottom & glued another inside, so it can stick over the end of the seat. When not in use, it folds flat. With some parachute cord & a bungee or 2, it can be filled & things strapped to it,you wouldnt believe all the stuff Ive carried..
For those who arent real good at knots, see the figure 9 tool.
Learnig a bowline is easy & very handy, I can teach it the fastest way to tie it,if you can make a slip knot, you can tie a bowline.