This is my 2x4 cart that I use to store and roll around the garage I'm sure you could just buy bigger caster wheels for it to run on rough ground
2x4 59" x2
1x4 59" x1 (center support that holds the window)
2x4 30" x2
2x4 5" x4 mounted 10" from outside
And then I put some carpet scraps on it for padding
5" between the 1x4 and 2x4
Window goes down, windshield wiper between the stacked 5" 2x4s. Window will open slightly and rest on the 1x4.
Wish I had some pictures of it sitting on there but I don't at the moment sorry.
Don't know if it's the plan you are looking for but it's a plan none the less