Some data...
+1 for Carl's and Gary's comments.
I recall reading a Bob Axom post from long ago, so I decided to tinker a bit with my -7.
Experiment:
I covered all of the rib ends with HVAC aluminum tape to create a smooth surface and overlapped pieces such that they wouldn't bind the flight controls if they came loose.
Aesthetics aside, I found that covering the inboard elevator ribs (E-705, E-709) yielded the greatest speed gain of ~2Ktas @ 6500ft. (1.75Ktas, 3Ktas, 2Ktas, 1Ktas, etc.)
All of the other counterweight "horns" and rib end coves didn't yield anything measurable. I suspect this is due to the angle that the elevator ribs are relative to the airstream causing a low pressure "suck" at that location.
...Steve Smith please opine...
Finish Technique:
I used 1/2" Owens Corning foam board for the filler material. I created a 3/16" radius around the edges of the foam so the rib would nestle inside the rib curve and provide a lip of sorts for the epoxy + filler to hold the rib into place. Glued the foam into rib "cove" with West Systems epoxy on the inside and West Systems Epoxy + milled fibers (fiberglass, not cotton flox) on the outside. Finish sanded flush with the skin edge. Drilled a small (#27 drill) hole into the trailing edges and bottom rib/flange holes to allow for water egress.