I use a thin tankbag , oxford sports humpback panniers across the seat but supported on 2 home made frames inspired by an unknown brand , and a rolltop bag across the the seat with the ends resting on the panniers .
this set up has served me well for several years , with the ability to carry enough stuff for 10 days away abroad .
I don't bother with bungee straps ( too unreliable ) but I do use releasable big cable straps . thee are much quicker and will not come undone and are far more secure .
the pannier supports bolt to the rear exhaust hanger rear bolts and the rear bungee strap bobbins ( gen 2 ) , I got the idea from some highway hawk ***** the Harley owners use , mine are made from bent and welded rebar rod , and have proved very rigid and protect the panniers rubbing on the zorsts ,, !!!!