It's a fairly normal thing for scooters/motorcycles, it could be pretty well done from the underside through a back rack, with blind "carriage" style bolts to the interior of the case as shown.

Sometimes carrying a backpack just isn't an option. And it sucks having wet gear inside of a backpack, you can bring plastic baggies, but it still sucks.

I guess it depends on the "city coefficient" for what gets stolen and all that, but it seems like $20 might be worth some tests. And if that's not super secure, I've thought about a more metal type box that could work. This would add a good 5 lbs minimum weight to the bike

If going/gone electric, this dual-serves for battery compartment and the weight factor is mitigated with the electric help.