Trick from the RV and BD planes: Put a curved bit of sheet metal on the bottom of the firewall. The idea is that it is hard for the slow air inside the cowl to suddenly accelerate around the sharp corner at the bottom to meet the high speed air just below.
You see this on many classic planes where there is a huge flare on the bottom of the cowl needed to eject cooling air during climb out. If you look just after the cowl opening the sharp bottom edge of the firewall is acting as a reverse scoop.
The RV builds in a cooling ramp on the bottom. It adds an inch or so on either side on the bottom of the fuse.
On the BD-4 where there isn't room for a ramp, the trick is to add sheet metal on the bottom of the fuse that sticks forward and curves up into the cowl. That way the bottom of the cowl can be left flat and still eject cooling air with less drag.
BTW, I'm jealous, your plane looks very nice.
Cheers,
Mark