I think you'll need a good bit of wood working to make that wrap-around into something that can take the heat of MH
basicly, I see a custom cut wood "box" that fits inside and is attached to the plastic 'shell'
then you can sandwich some asphalt or fiberglass insulation between the wood and the plastic
inside of the wood box, you install your reflectors and lights, as well as cut 2 or 3 holes in the 'top' of the box to install some vent fans.
basiclly you just need some 1/2" plywood for the box 'top' and bracing, some 1/4" plywood for the box 'walls', a jig saw, some nails, wood glue and squares of asphalt insulation or a small roll of the unfaced fiberglass insulation (get the formeldahyde free stuff), and a good long xmas vacation
i would scribe the inside dimension of the shell onto the plywood, and then cut that line, to keep the good bow shape. then take 2 pieces of scrap 1/2" plywood and make some "ribs" that go from front to back across the top so that you have something to help keep the 'wall' bowed. once the ribs are fastened in place, you can shape the 1/4" plywood to the bow, as well as attach the sides and back of the 'box'
once it's all glued and nailed, it should fit nicely inside the plastic shell ... then it's a matter of a few well placed stainless steel screws to attach the two together. (paint the screw heads black when you're done
give it a day or two for the glue to harden, and then you can cut the holes for the fans, and install the lights.
the wood combined with insulation should protect the sides of your shell from warping, and the vent fans on top will help keep the tank from heating up too much from the lights
sorry it sounds like a lot - if I could draw worth a darn, I'd do some sketches for you, it looks a lot easier than it sounds I think (i just see it drawn in my head, making my hand follow that is another story!)