First do you have the right size of tank? An initial estimate would be 2oz of capacity for every 3” diameter cannon, 4oz of capacity for every 4” diameter cannon, and 6oz capacity for every 5” diameter cannon. This first estimate should completely empty the cannons with enough gas for about another half a magazine load of ammo. This means you do not need to refill the tank between your first and second sortie, just the ammo. Of course the size of the ship may force you into a smaller/lighter tank.
Typically the tank is located near the center of the ship either between or on top of the batteries. The tank should be mounted so the valve end is inclined slightly. This helps to prevent liquid CO2 from getting into the cannon, which could cause a dangerous overpressure condition. The tank must also be on the centerline or it will cause the ship to list as it empties. Install the CO2 tank near the center of the ship. Otherwise as the CO2 is used and the tank gets lighter the ship will take on a bow down or stern down list. Install the tank such that it can be easily removed from the ship for refilling. Removing a dozen screws to fill the tank is a real time waster on battle day.
To build a mounting platform trace the outline of the diameter of the tank on a piece of 1/8 plywood or plastic and draw a line across the middle of the circle. Pick two places where your stand will rest either on top of the batteries or the hull. It is best if they can be attached to the hull. Mark these lengths on either side of the line on your stand pattern. Make the stand as wide as practical and at least as wide as the tank itself. You can get creative and make a chalice type shape for each stand in order to save weight and space for plumbing. Next glue the stands centered on the hull. The stand should not impede the flow of water through your water channel. You can use hooks and rubber bands to secure the tank to you stand. Most tanks with an anti-siphon tube have the tube pointing towards the on/off knob on the valve. So when strapping in the tank make sure the knob is pointing up.