Jeremy,
This may not transfer well to your Burro, because the inner shells in the later ones like ours join together with the outer quite close to each other,
Our strip is only 8" wide. What they did was glass in a number of small plywood squares into the recess, then make the cover of a long strip of thin flexible plywood covered with a thin layer of batting or some sort of cushioning material, then wrapped this in white vinyl and stapled it to the plywood on the backside.
The whole strip is then screwed into these squares and the screws are covered with white snap-on plastic finishers (widely available).
There are some advantages to this: 1) You can remove and replace the whole thing in no time flat. 2) The plywood holds the shape well. 3) Access to wiring and whatever in the recess is easy, including drilling into the
fiberglass inner shells to put in new wiring, coax for antennas, etc.
Because of these advantages you might consider changing from the original glued-on method to this later method, but the wider strip necessary may be a a problem. Maybe more plywood squares, using a double row of screws instead of a single row perhaps. Just leave enough room around the edges of the recess for wires.