just a regular on off switch works. I used the left side tank as my default tank and that is the tank that is selected when the switch is off. (chose that tank cause its the side my truck tank is on). then when you turn the switch on it selects the other tank.
just get an on off switch and mount it side ways. so that when it is to the left it is off. there by selecting the left tank. when you turn the switch on or RIGHT it selects the right tank.
only issue is if you leave it on the on position there will be a tiny draw on your battery to hold that switch in the on position. But a car batter will probabaly last for weeks if that switch is the only thing drawing power. But if you turn your batteries off when ever you park your boat you will never have an issue with this.
this works great and it is the system i have used in my get boat for years. So i can vouch that it works fine.