Is rmax insul board? If so, maybe glue directly to the metal and use a t-square to mark the void locations. Drill a 1/4" hole every foot or so up to the top. Then use LOW EXPANSION foam (the stuff made for around doors and windows so it doesn't bow them, there's a huge difference between that and the regular stuff.) I'd still put a temp frame against it even with low ex. And probably use a rustoleum primer or an epoxy coating. You may also be able to avoid the moisture by completely sealing and gluing the board and putting a vent at top and bottom of void, but do so in such a way that moisture won't enter from driven rain or rain splash. If you try this, make sure you prime after cutting the vents. This is the same concept used on most roofs. You have an eve vent and a ridge vent. Behind the insulation (on back side of roof) a channel similar to those voids called proper vent is installed. This creates an airflow from eve to ridge of the same atmospheric temp as on outside of roof. Because of the low differential temp, this prevents condensation on the plywood and under the shingles. Which, in colder climates, would result in the condensation freezing and lifting shingles. And prevents the heat from inside melting the snow. Which compounds the problem as it gets drawn in behind lifted shingles when the melted snow turns to water then refreezes and expands, getting pushed up and in.