Where do you need the card? Doesn't you computer have mic input? Or are you using it as recorder?
It may not be possible in the space available but short, stiff and light with a negative preload is very effective at eliminating hammer bounce. The downside is that all the cocking force is applied at the 10-20 percent of the lever stroke.Tom
Tiivistekeskus sell o-rings on metric and imperial sizing. They have very good selection.