I agree with the baseplate approach. You should note that the Brunton 8099 and a Silva or Suunto baseplate function differently for map use. The Silva/Suunto designs have grid lines on the rotating capsule which makes some aspects of map work easier (IMO), the Brunton does not.

I have the Brunton 8099 and have liked it very much.....except every time it has gone above 8000" it developed a massive bubble which because it uses a rotating compass disk, made the compass unusable (disk would not turn).

Both times Brunton replaced the capsule, but I do not trust this compass at altitude anymore. BTW none of my Suunto or Silva's developed these bubbles on these trips, and even if they did, both use needles not disks so would have functioned.

Brunton assures me that the new liquids are more stable at altitude and temperature variations.