Before you install a plenum, consider all the things you can do to reduce cooling drag leakage.

First get the upper soft baffling as tight as possible, but you knew that!

Get some common window/door foam at Home Depot or the Auto Parts Store (I am using both) and stuff or silicon it in place.  I am using it between cylinder bases, around baffle edges, etc..  Use a light under the cylinders and baffling a dark hangar to find small voids.   The foam has been unaffected by heat for a couple of hundred hours.

Use springs to pull the lower cylinder baffles together tightly.

Caulk the baffle seams that are too tight to get foam in.  Use a paste wax on all surfaces first to make it easier to remove when necessary.

A difficult soft baffle seal at the front of the lower cowl is made much easier by glassing a little shelf as a extension of the aluminum baffle only attached to the cowl with a 3/8” clearance, sealed by a flat soft baffle.  (Thanks Leighton Mangels for this idea)

I don’t use a blast tube for the Gascolator.  My thinking is fuel does most of the cooling.  My hangar mate even blocked off the Cabin Heat (that’s a big hole) and removed the Heat Muff for summer flying.

If you take your time and keep tweeking the soft baffling (or scrap it and start over) until it is TIGHT you will probably have reduced cooling drag significantly. 

Our Legacy has always had cool oil and cyl head temps..  As in partial oil door settings (another drag reduction) for 175 degree oil at almost any cruise power setting above 8000’ and cyl temps in the low 300’s year round. 

Steve Colwell  Legacy 550