Al, Here are the links for the vents I used like you asked. The side vent I used for exhaust is 21" wide.
Just a note on the AC mounting. I wouldn't worry about the AC "bouncing around" regardless of what height you choose to mount it at. Just screw it down to the shelf you build for it. It will be fine. Use 4 "L" brackets into the AC and then into the shelf. The shelf supports the AC vertically with L brackets holding down...the "L" brackets will keep it on the shelf. Additionally, the main forces that would dislodge the AC are lateral forces (side to side)..... and if there is enough lateral motion to dislodge the AC from the "L" brackets... you have much larger problems.
The Side vent used for Exhaust
https://www.amazon.com/gp/product/B004L ... UTF8&psc=1
Top Vent used for Intake:
https://www.amazon.com/gp/product/B0019 ... UTF8&psc=1
Base for Vent Cover
https://www.amazon.com/gp/product/B002O ... UTF8&psc=1