Hi
There are companies that specialise in this type of thing, but not sure if there is a minimum order, I remember driving past such a company a few months ago, I can't for my life remember where it is though. A look on the net will probably bring something up!
Are you alowed to put propper shelters in an allotment, and grow on top of them?
I am sure I have seen companies remaking shelters.
Cheers
Dave