When by myself i use a big iron rest off drivers door.
Out passenger side is a length of 42x35 (about 1300mm long from memory), with carpet wrapped & taped over one end, & a bit of foam rubber pipe lagging taped over the middle third.
I lift this up so it rests from dash (carpet end) to shoulder of passenger seat, & rest the forend on this...so far (touchin wood) ive not shot out the passengee side mirror on a car since the first one, some 25 years...
I find what you need all depends on the car you're shoot'n out of...some cars (like my Triton) are a pig to shoot out of, & the mirrors are so high & big they restrict your shooting arc by half...
Best to sit in a car with a rifle & see what you need i rekon, cos window, seat & mirror heights fluctuate enormously between vehicles.
Tell your mate pumpkin to give up shootin & take up netball & leave the hunting to us men...!!
The man who knows everything, doesnt really know everything...he's just stopped learning...