Trial fit the fuel tank. Leave plenty of room for the necessary plumbing. (If using a YS engine, you may wish to consider fuel filter and air check-valve placement, if placing inside the fuse; as well as the width of foam padding. I forgot to consider these additional factors and later realized I placed my servo mounting rails too far forward to mount everything without risk of kinking the fuel lines for the Tetra tank I was originally planning to use. Luckily I had an old Hayes tank that is shorter in length that I will be able to substitute, instead.)
After determining proper placement of the tank and fuel system, cut plywood servo mounting-rails to the width of the fuse, where placement is optimal for the equipment being used, somewhere behind the fuel tank. Trial fit, with servos in-place, to ensure servo output arms have plenty of clearance above the wing saddle. When satisfied with equipments’ placement, maintaining appropriate clearances, glue the servo mounting rails in-place.
Center the Elevator servo’s output arm as close to the centerline of the fuse, as possible. Position the Rudder and Throttle servos on the outsides of the rails, as necessary. (Consider the Throttle servo’s output arm in relation to the position of the carburetor of the engine being used.) Reversing/alternating servo orientation helps to minimize risk to interference of the output arms.
When servo placement has been determined, glue in tri-stock to reinforce the glue joints of the mount rails to the rest of the fuse structure. (Ensure tri-stock does not interfere with desired servo positions.)