One thing to keep in mind is plenty of ventilation.
My son's PC was fried by heat buildup in the cubby hole of a so-called computer desk. He kept the PC on all the time and would close the door to the cubby hole. The only ventilation was a 2\" hole in the back for wires and it was filled.
You might want to get the customer to sign off on a statement saying they understand the problems heat buildup can cause and you are not responsible for any damage to the equipment due to heat.
I had my computer tech wire a small fan about 4” in diameter to my computer so when I turned it on it comes on automatically. We drilled a hole on the side of the cabinet to mount the fan, it works real well with the door closed. Also put a few holes in the floor in front of the toe space to draw air in front of the tower.
You might want to get the customer to sign off on a statement saying they understand the problems heat buildup can cause and you are not responsible for any damage to the equipment due to heat.