I just measured the area behind the servo (Spektrun 6040) to the battery brace of my 22 in rear engine configuration and found the following dimensions:
For/Aft (servo to battery brace): 28mm
Side to side (flat portion of chassis): 56.5mm
Height of chassis plate to body: 40mm
So the area to mount reciever/transponder etc. is 28L x 56.5W x 40H
The front windshield starts pretty far forward so height over this area is adequate.
Speedo Tray mount Dimensions (Over Battery) in Rear Engine Configuration:
38mm L x 43mm W x 37mm H
Hope this helps!