You can glue a servo to plywood, HOWEVER, there are a couple of things you need to be aware of before doing this.

1. 99% of the time this is a PERMANENT thing. If the servo goes bad, forget it.

2. Be sure you use a GOOD adhesive .

Since you will be most likely laying the servo on it's side to glue it in, why not make or buy some "L" mounts for the servos ? They are very easy to make from some 1/4" basswood. Screw the servo to the L mount, then screw the L mount in position. They use no more room than the servo itself.