Gday,
Have fitted servos side mounted without any great problem. As previously mentioned, if it mounted too low you may have trouble with horn arm length. Pics below are for my Top Flite spitfre wing. Servos mounted on side using hardwood rails and aluminium servo bracket in an 'L' shape. Servo is screwed to bracket and bracket to rails. Top side of servo just (hair width) under the 1/16th ply hatch cover. The servo is a Futaba S3010 and using an aftermarket heavy duty horn which is slightly longer that the ones supplied with servo.

ATV on radio set at 100% and throw of servo adjusted with dual rate settings at 50% and 70%. Arm moves front and rear without catching on hatch cover.

Regards
Mark