Insides trimmed, painted and covered with weatherstripped plywood lids to keep the weather out.
From the dirt surrounding the lockers in the picture, you can tell pressure washer weekend has not yet happened (but soon). There is a ventilation port on the inside (can be faintly seen in the picture, to the right). This is to provide engine room ventilation. Which reminds me, with all the insulation inside, and the summer weather, things do tend to get warm. I need to get a draft assist fan installed at the opening there.
The storm covers are clamped in with a single bolt to give the weather stripping a good seal.