I have been experimenting with old table cloths and bits of tarpaulin, to see how a cockpit tent might work. The answer is really rather well. There could be a central "flat roof" section, between the back edge of the spray hood and the boom gallows. Then a simple wall on either side , which could be rolled up easily. The internal space would be very usable because it would be high for its whole width. There may be a risk of rain getting in through the roof, which would be fairly flat, but I think it could all work. If made of a thin canvas, it might even roll up and be stored on the gallows making it quick to fit and remove. I have had a rough quote of 400-500 pounds from a sail maker in Poole for a tent of this rough size.
What I am planning to do is to have two new triangular windows fitted in the sides of the spray hood. They are terrible blind spots if you are trying to shelter and sail with the hood up. I have had a quote of 64 pounds for both windows, which I think is good. Less than the cost of a tank of diesel these days. I have marked the windows and just need to find a box to pack the hood into.