You dont even have to modify your backpack, get some good straps with a cam buckle, 2 will do. And just lay your backpack on the top of the rack. no modifications and same amount of space you had before plus you get to just carry a normal back and not a pannier. Otherwise Topeak makes a laptop specific trunk back, which might work for you. If you really want panniers go for ortlieb, the most waterproof, best on off system and they hold everything youll ever need with room to spare.