We realized early on that horizontal members were needed as well to provide support for the Masonite. We had already placed a horizontal 2 x 4 at the top of the wall so the "L" channel around the perimeter of the suspended ceiling would have something to attach to. We also added a horizontal member at the bottom of the backdrop and one midway between the top and the bottom. We cut these horizontal boards as carefully as possible so they were a press fit in between the vertical members. We used the molasses-colored Gorilla Glue at the joints, which expands as it dries if you have pre-wet the mating joints. This eliminated the need for screws or other hardware which could get in the way of attaching the Masonite.
I fully intended to paint the backdrop in the new room using the same technique...until our gallon of the darker shade of blue ran out. The original color was discontinued and even though we had the formula, the mixing color palate had also changed. Two attempts to scan and color match the old shade were disasters and the thinner consistency of the new paint did not blend well with the thicker consistency of the old paint we still had for the lighter color. After painting the backdrop in the new room twice with very poor results, I made the decision to paint ALL of the backdrop (new room and old) a single, uniform color. I don't regret that call for a minute.
I initially bought a 12-inch diameter cardboard tube at Home Depot, which we cut in half lengthwise. It was about 1/8" thick, the same as the Masonite, so I just glued the edge onto a vertical 2" x 4" at each end, intending to put the Masonite up against it. I tried to fill the gap with drywall compound, but it kept cracking. The cardboard was not very rigid and every time it got bumped, the joint cracked. In addition, we were trying to make a 12-inch diameter tube stretch to 12-3/4", which it did not want to do.
I finally went to a concrete contractor supply business and bought a 14-inch diameter tube, which was much heavier and a full 1/4" thick. We built out our framing another 1-1/2" and that was a better fit. Using a router, we then cut a groove in each of the framing members, 1/8" deep (half the thickness of the cardboard tube). We applied Gorilla Glue in the grooves and stretched the cardboard tube just a bit to fit into the grooves. After the glue dried, we had a continuous joint that was strong and ready to mate with the Masonite. As with the joints between sheets of Masonite, we left a 3/8" gap which we filled with Bondo.
The cardboard tubes are spiral wound and the seams show if not addressed. I used several thin applications of drywall compound to minimize their visibility. After painting, they are barely noticeable.