Probably the easiest way is to cut a hole in the side of the fuse..laminate some GOOD 1/8th ply in front and back of the holes to the wood,It needs to be the 5 or more ply, not that light cheap 3 ply... remove the covering where you glue the ply on. Place servo in hole,drill the screw holes..take out ...harden holes with thin ca, plug servo in to extended wires from reciever..buld your pushrod setup..THIS ROUTE WILL ADD TAIL WEIGHT, so be sure to balance...

This type of setup is good for the rare plane that builds nose heavy. There are trickier more complicated ways it can be done..mainly to hide the servo....

personally I would never go smaller than a standard servo on the elevator of a 40 size bird. I would place the servo in the fuse so that I have about 5-6 inches of pushrod ...it can be done shorter....