Hi there, This is something that I had to cover (pun intended) this year with my 3.8m Honwave. It gets folded up and put away in the winter but around March - April It gets inflated and put all the trailer all summer.
I ended up getting a PVC tarpaulin from Clas Ohlson. It is perfect, totally waterproof and tough and has eyelets every 50cm all the way round. I think you can get a 3.5m one for about £30,worth a look to see if they have a store near you or find out what the delivery charge is online.
Whoever you buy from make sure it is PVC and not a polyethylene tarp.