No guarantees any of these are obtainable in Singapore. You may have to order them online and get them shipped, which will not be cheap.

Timbuk2 Especial Cuatro
Cycling Laptop Backpack 2015 - Especial Cuatro Backpack | Timbuk2 Bags
Giant, expandable, waterproof materials, reflective bits all over, air channels and ventilated mesh.

Boreas makes some semi-modular packs where the pack is suspended on a frame with a mesh back, such as the Bootlegger.
https://www.boreasgear.com/collections/packs
I haven't personally use the Boreas packs, though.