Spray foam the ceiling... the walls too if want to heat it a little. The width is the width, unfortunately. Smaller vehicles fit ok.No way I'd go with a sea-can... I hear too much about sweat issues and too narrow for a vehicle. Plus, it would cost probably as much as framing out a small enclosure; my hand notes for lumber came in at a little under $4k to frame out a 24'x16' shop with small bonus room on top(no truss/roof sheeting) Last 20' sea-can I priced was near $3k with no delivery.
I don't have room for a container, but one buddy has several now. He has it figured he can't build the equivalent with lumber and concrete for less money.