If you just need to cover your rear spoiler and not a wheeled assist, I use a plastic roll of non-adhesive shelf protector held in place with a magnet that has clips, I have covered the magnetic part with thin cloth glued on so as not to scratch my car’s surface.
https://www.staples.ca/products/135382-en-staples-magnetic-bulldog-clip-2-14-3-pack?CID=PS:GS:CA:::::pla:21473839577&CampaignID=21473839577&gad_source=4&gbraid=0AAAAAD_dfLp3eLXmr0xdvvNhc6k_PQprm&gclid=CjwKCAiAneK8BhAVEiwAoy2HYZDENFvHNr541WPV8-N66hNcNfp_BlUZcvZhV7S5ziV3diDqn1HiqBoCtg8QAvD_BwE
Con-Tact Premium Non-Adhesive Plastic Shelf/Drawer Liner, Frosted, 18-in x 4-ft 9.99 at Canadain tire
This stays in place. Previously I used a bath mat with a rubber grip but it got moldy left in the car on a long summer trip.