This comes up from time to time and you will get varying answers if you search including Van?s standard respsonse of ?it?s not necessary?. This is why they have historically never supplied one and don?t sell one unless they have changed their mind recently.
Many of us don?t use a gasket between the airbox and carb or servo. It can?t hurt, but there is little value to using one in my opinion. My opinion is always subject to change however.
Whatever you do, do not use any type of paste or other sealant. If you feel you must seal this joint, most definitely use only a gasket made of suitable material.
In flight, ram air is going to overcome any kind of leak here. I would be curious to see if at idle, air can get sucked in. To find out, you could do an ether or stater fluid check, but I am not that brave!