Here's what I've done.
The lower trigger part, I've used BIG's method.
The space behind the trigger mech is 50mm in lenth.
The bolt moves back around 4-6mm into that 50mm gap.
Note that if you do fill the all 50mm gap, you will reduce most the recoil, and have a longer lasting hinge plate, and result in short stroking (bolt won't lock back).
I've took foam cusions from Shooters Design slide packaging, neat red too.
I've cut it so it's 46mm L x 24mm W x 26mm H .
Here are the pics.
Also, to reduce the shock, like Airsoft Surgeons method for hi-capas, you can probably find a rubber washer and throw it on the recoil spring rod. Not too big, but big enough for the spring to fit through the washer with the rod. Just a thought.
