As pinballlizard points out, a loose tarp can do much damage.

Personally I like to transport pins in an open bed without any cover on them. I have not had any issues with plastics melting in the sun, but I guess it could happen.

If you feel that you must cover it, I would strongly encourage you to use a bed sheet or something cloth like and wrap rope around it like crazy to keep it tight. If you have a 3 hour plus drive any type of cover will be trying to work it self loose and it will unless you do an excellent job securing it.

I much prefer transport in a station wagon if it is only one pin. Low to the ground, easy to load and protected from the outside.