A shop that makes covers of various descriptions for boats might be able to help.
Plastic tarps are cheap but useless. You will spend more replacing tarps than a good quality marine grade material in no time.
The question is, how to do it? Some sort of snap on bib might be the answer. Done right this could stay on during travel if you wish. The hard part is reaching the snaps on top.
If you consider this route DO NOT let it be made super nice & tight. Impressive when first made for neatness & all but over time things shrink & next thing you know it is a real struggle to get things fastened.
I have dealt with more than a few marine applications in my day.
Boat: 32' 1996 Albin 32+2, single Cummins 315hp
40+ night per year overnighter
2007 Alpenlite 34RLR
2006 Chevy 3500 LT, CC,LB 6.6L Diesel
Ham Radio: VP9KL, IRLP node 7995