I use the following which have been excellent:
Halo Tubephones
Given the extremely limited space in our F1 racing aircraft I had no room for a bone dome or similar so carefully cut the halo from its frame and used epoxy to build it into the shell of a Carbon / Kevlar / Dyneema skydive helmet. Its fair to say our aircraft are extremely noisy and this system works very well indeed in blocking out the racket and is extremely compact while still offering some reasonable protection.
In hindsight you could actually just leave it attached to the frame and wear it under a helmet however I wanted it as faff free as possible. Used it like that for 3 years now without issue.
Link to helmet I use and no I'm not on commission from either!
http://www.carbonphonix.co.uk/index.php?p=1