After getting the front and back sorted, I decided that actually the back wasn’t sufficiently high, so I made the sides a little higher and I’ll add an “extension” to the back to match them afterwards.
After extending the back I added the timbers for the roof and wrapped everything in breathable membrane. To make it mostly watertight (there’s no door yet) I found some scraps clear roofing and fixed them down. They’re really in no condition to use as a permanent roof though. I just need to work out what I’m going to make the final roof from.