Well, I'll show how it is w/ the Nelson Rigg bag. It's not an elegant solution, but it is one which does not require any sort of mechanical aptitude. It can easily hold my shoes and lunch and I can squeeze my macbook over that if I don't have a case.
It's secure but a bit of a pain placing the Underseat attachment. I had hoped to place both of attachment straps under the pillion seat, but could not get the seat to lock down, so I had to settle for one under thepillion and the other a bit more foreward.
The way it is now, I can fit my leg over but ideally I'd have it scooted back a few more inches.
Because straps with male/female buckles are all attached to the bag it is a bit of a pain taking it on and off the bike as you have to feed the straps through the D-links attachments each time so likely I'll use it for lunch and shoes and leave the bag on the bike. I can carry my macbook and keyboard in a sling bag.