Stefan,
I am not sure the Be windows are tougher than
your polymer ( kaptan?? ).
If you are looking for solutions, I am wondering
if the solid state detectors like made by AMPTEK
(www.amptek.com) or Contech (www.contech.com)
could be retrofitted. The Amptek are what was in
the "nose" of the Martian Rover. These seem to be
in the $2500-$5000 range. These detectors are
small but you might be able to get them closer.
I am guessing you are replacing your own windows
because it cost us about $1000 to get Ortec to
replace the Be window + refurbish.