I did it for many years 75-85!
To protect from scratches & pine pitch on the paint - I just put a tarp or something in the rear trunk & over the rear fenders & rear wall, put the tree in sideways & folded the tarp down over it, then tied the trunk closed with some string or a long shoelace threaded through the upper & lower latch parts.
Then drive slow & don't get too tall of a tree!
I also carried my bicycle in there that way, with the front wheel removed for mor compact fir - that is, when I didn't use the bike rack from the under rollbar mounts & trunk pieces.