Some ready-made covers are available (Fiamma for instance https://www.fiamma.it/index.php?route=product/search&filter_name=Bike%20cover ) but check dimensions.
If ready-made ones don’t suit, an alternative is to have one made to your dimensions.
When I took a Trials motorcycle on the back of a motorhome some years ago, I commissioned one of the windscreen cover companies (SilverScreens I think) to make one to my dimensions. I would suggest that, if you cannot find a ready-made one that suits you, commission one of the windscreen cover companies to make one. A search “motorhome windscreen cover” brings up lots of them.
Good luck!
Barry