Gun Troughs:
The gun troughs sit at the top of the engine accessory bay, although they are largely hidden beneath the uppermost cowling. They form the lower half of the channel through which the forward-firing .50" machine gun barrels pass. While the gun troughs may be the smallest sections of engine accessory bay cowling, they are perhaps the most complex. Their multitude of compound curves make them a challenge to manufacture. The project arrived with a set of these cowls, but they had to overcome significant issues on the road to airworthiness.
The Lefthand Unit:
The lefthand gun trough suffered from corrosion, dents and cracks - not to mention a hasty wartime repair to its aft end (see right side in below image). Pioneer's engineers exerted considerable effort to address these issues. They replaced the wartime repair with a structurally correct example, and  spliced in a replacement section of the aft rib to account for a crack in one of the mounting holes.
A view of the lefthand gun trough in the accessory cowl which reveals the aft rib prior to restoration. As is evident, there is a crack through a mounting hole. The outboard length of this rib (right side in image) ended up being replaced with new material, spliced together with the salvageable section. (photo via Pioneer Aero Ltd.)
The section of skin from the wartime repair following its removal. A new internal doubler was manufactured and fitted prior to fully-removing this damaged section of skin. The doubler was then held in place with clecos while replacement skin was manufactured to duplicate the wartime repair, albeit using a structurally sound technique. (photo via Pioneer Aero Ltd.)
This image shows a test fitting of the aft end repair to the lefthand gun trough (clecoed in place) during the manufacture of various replacement components. The new aft section of skin is evident (righthand end) along with a length of replacement material for the aft rib (the end of which is visible through the opening). (photo via Pioneer Aero Ltd.)
The Righthand Unit:
The righthand gun trough was in an even more distressed condition than the lefthand unit. It suffered from corrosion, dents, and cracks along with a partially crushed section as well. Pioneer’s technicians repaired all of this damage but had to replace the aft rib, which was cracked beyond use.
A close up of the crushed section in the righthand gun trough. It is partially straightened in this image, but there are multiple cracks on display. The damaged section of skin had to be cut out and replaced with a doubler around the edges and a skin insert on top. (photo via Pioneer Aero Ltd.)
A view of the righthand gun trough showing the test fitting of its replacement aft rib (the end of which is visible below the clamp). The new rib is attached to the firewall and awaits the drilling of rivet holes to match those in the skin. (photo via Pioneer Aero Ltd.)
A view of the right hand gun trough following the completion of most repairs. This image is part way through the process of riveting on new material to replace the section of skin crushed and cracked during WWII. The remaining repairs were riveting into place following the capture of this image. (photo via Pioneer Aero Ltd.)
The righthand gun trough following the completion of all repairs and the installation of new Dzus springs. It is marvelous that so much original material could be saved! (photo via Pioneer Aero Ltd.)
Test Fitting:
Pioneer's technicians trial-fitted the two gun troughs into the engine accessory bay cowling assembly to verify that they would connect correctly. Given the level of preparation prior to this moment, it should come as no surprise that everything fitted exactly as it was supposed to.
Following the completion of their repairs, both gun troughs are trial-fitted into position atop the accessory cowls. (photo via Pioneer Aero Ltd.)
A view of the completed righthand gun trough trial-fitted into the accessory cowls. (photo via Pioneer Aero Ltd.)