I'm a squashy bag kind of bloke too, but you can get some nice custom made flight cases for under £200.
Custom Flight CaseThose flight cases are flippin' excellent for lugging stage gear around the globe, but they might be a bit overkill for carrying your spare shreddies on the back of a Morgan. Even empty they weigh a ton, the sides are around 7mm thick.
I use a squashy bag behind the seats and a Peli Storm case on the rear rack if I need a bit more space.