Our cover is now in it's fifth winter. The first winter we covered the windows with cling film as advised but have not bothered since and have had no ill effects. Hedgerows and trees do more damage to windows than the coverdoes.
This is our 3th year with a cover, (specialised accessories) And like Maurice we used clingfilm but theres no need, and no problems, just make sure it's
a good fit, and use bubblewrap on the sharp corners all the best Jon.