Its probably not a great help but if you do want a TM P90 Hop Up Unit TMC has them for the same price as CA ones, HOWEVER they are out of stock. I just emailed them about ETA for a new batch so hopefully they get back.
http://www.tokyo-model.com.hk/ecshop/goods.php?id=276
I didn't think CA P90 Hop ups had any installation trouble or this problem in general.
Also found this about P90 Hop Up Chambers...
http://www.airsoftretreat.com/forums...;topic=86580.0