I’ve been using that red Target backpack—it was packed to the gills yesterday and incredibly heavy. I just sit it right on the rear rack (not lying flat, mind you, actually sitting upright), loop the straps around the seat post, tie off the excess so it doesn't flap around, and let the bungee cord from the rack hold it steady.
The thing is, once it gets too heavy or overstuffed, it starts acting up—it doesn't sit quite right and tends to tip whenever you hit a bump. The straps start wiggling and catching on the tire (it never snagged when it was lightly packed), so riding it was a bit tricky—maybe adding an extra bungee would make it more stable, I suppose.
But when it's filled normally, it behaves perfectly and hasn't fallen off once; I've ridden through Central Park with it, and even did a quick stretch up Dotki Hill—though it gave me some trouble there because the shifted center of gravity tried to pull me downhill, forcing me to shift it onto my back. I wasn't exactly racing, but we were definitely bouncing over tree roots.
Personally, though, I wouldn't dream of putting a laptop in a setup like that. I just don't see it working—besides, the laptop would likely bounce around way more than my own back or the front of the bike. 🤷
And let's be honest, a laptop is a much more expensive little gadget...
Anyway, what do you think about a carrier like this? You might even be able to squeeze a real laptop bag inside.
Check out the "BUSY BASKET Basket and Carrier."
From what I can tell, it's an American-made product, but I couldn't find any info on its weight capacity.
http://jupiteriana.com/hr/busy_basketWell, you could always give them a call and ask. 😉