40x30x15cm British Airways hand bag recommendations

Hi, looking for recommendations for a decent backpack that meets BAs rather stingy 40x30x15cm hand bag allowance for a long haul flight. All it really needs to do is fit my 15" laptop and my dSLR with a 70-300mm lens attached. Most the laptop backpacks I've seen are too tall. I did find this backpack by Aerolite on Amazon but some of the reviews say the build quality isn't great... https://amzn.eu/d/eiy2gEv, preferably something <£50. Thanks :)