Soffit ventilation!! @aart can tell you more about it. Because the roof is already transparent, you don't need it to be transparent all the way around, but you could, as @3KillerBs said, leave a side open but covered by hardware cloth.
Because it's already roofed, I think that you would just need to cover up the large gaps in the walls and seal for drafts. I know that T1-11 is good siding, you could get some of that and put it on?