Hi everybody,
@tony27 :
my bike IS a 250...
@pschrauber :
yes, I've used styrofoam, shaped to use the maximum of space in the frame (with the possibility to remove the assembly airbox/inner guard without removing the shocks and/or the exhaust), then it has been recovered with fiber glass and epoxy resin, painted, sealed and riveted to the inner guard.
i've also made a heat shieldbetween airbox and exhaust :
Please note that i've got a dellorto (28) and a WES exhaust system. the genuine airbox was definitively too small for well breathing...