Portable 4 place intercoms-roll your own?

I was looking at these, in anticipation of getting my ticket soon (I
hope!). The prices seem to be all over the board. Many seem to have an
add on module for use with 4 passengers.

I certainly don't know how to design electronic stuff, although I can
read a schematic and build something from one, so how hard is it to
build your own portable setup for a renter? a battery or running off the
planes 12V socket to boost the signal somehow (I guess-many units seem
to require one or the other). A bunch of jacks for the headsets, a
couple of plugs for going into the pilots com jacks. Switches for
all/pilot-copilot isolate/pilot isolate (sounds like a mess of wiring in
their), maybe auxilary inputs for CD players and cell phones.
Hmmm...how do you wire in the copilots mike when the plane has built in
PTT switches?

Starting to sound more complicated than I thought.

John