Good morning Barry .
Yes BUT it will take a lot of money and time . The gasifier design is designed to use very cheap 55 gallon drums and lids with good seals . If one has to fabricate larger containers that will seal or lock he is talking money .
One can lower the height of the gasifier when going through the bed and mounting on the frame . With the dakotas you can lower 11 inches all other trucks I know of is 4-6 inches .
I have never had any pre gasification condensation ( hopper juice ) to freeze but keep in mind I am in Alabama . Drainage needs to be a vertical as possible for the removal of tar.
Post gasification condensate freezes as easy as water .
I believe as you get more familiar with gasification you will realize having the gasifier in a camper shell , box or cover will not be a good idea .