Here's the MQ-1 board mounted behind the ROTJ Hero keyhole (its placement is identical to all the helmets):
And the ESB Hero/ROTJ SE helmet:
These images shows the inside of the ROTJ helmet:
And here's the inside of the ESB Hero helmet:
The MQ-1 board is secured in place with some kind of glue or putty and the lower half is closed with a curved piece (possibly the same material as the visor) which is also secured with the same glue or putty.