I was thinking machining it out of a solid chunk of aluminum. Your sketch is great, but I'm not sure how thick the lower edge would need to be for the exhaust plate to be flush. I took a quick look on aircraftspruce.com for aluminum bar stock. They have ½ inch thick X 6 inch wide bar of 2024 T3 (same as sheet on pre 1969 trailers) for about $35 per foot. That could end up being a pretty expensive wedge, but the machining would be pretty straightforward.
I'm leaning toward the NT30 also. Seems like plenty of heat output for a 26 ft. I plan to put the plenum above the floor and under the new support I build. I'll use the single cold air return as a hot air duct and add the other vents from the plenum. I like your idea of leaving a ¼ inch gap, although, I may add ¼ inch insulation board. I need to raise the new furnace 6 inches, so with ¼ inch top and bottom and ¾ inch for the support, I'll still have enough duct height to easily handle a 4 inch duct. Thanks for you insights and experience.