If it is flat foam and you want to put it on top just use sticky pads and put tape over the top and down the sides to the wing.

Method 1:
If it is a flying wing set servo on top.
Drop line around with pen.
Cut with xacto that has tape around blade at the same height as the servo thickness.
Clear away foam inside square with small screwdriver.
Try to place servo in. The tabs will make indents on the foam.
Slice those with the blade. Slice where the wire wouldn't go in too. Slice out a part where the servo head and arm moves.
It all takes less than 2 minutes once you've done it a million times
Servo will fit snug in hole and thin clear packing tape over the top holds it in fine.

Method 2:
Use 100w soldering gun with wire bent shape of servo and bends to stop it going in too far.
Place business cards around lines and use gun.
Takes 5 seconds but awhile to get the wire setup right.

I can post pics.