Are you talking about a clear sheet of plastic that sits in the rim grooves and covers the entire tank? If so, get rid of it. Dont see whay you would need that with a full canopy like you have. Also, you need to vent that canopy with some fans if you havent done so or you may as well call your tank a lobster pot.
lol For one if you have a center brace you can't have a MH right above it.. Two your canopy looks very low.. Mh should be atleast 8" off the water and really when you first upgrade from pcs to Mh it should be much higher..Also Even if your canopy is 11-12" tall your going to need some air movement int he canopy.. Atleast a small 4" fan maybe two..
Light spread. Metal halide is a point source light. Depending on the reflector, you dont get enough spread throughout the tank without the reflector being raised a minium distance. 6 - 12 " above the surface is a general guideline but 6" is generally too low. If you look at several halide/relector spread patterns you can see that you dont start getting tank wide light spread until the reflector is raised a specific distance over the surface of the tank. So, depends on the reflector and how much area you need to cover (dimensions of tank) but generally 9 - 12" ends up being optimum. On a related note, heat is also a factor. So, if halide needed to be placed 3 or 4" above the tank, it wouldnt work out very well anyway. Any plastic would be toast. Plus, the hot spot (super bright area in the tank underneath the bulb itself) would be way too intense being that close.
Enclosed canopies need to be vented or heat is going to be a major issue.
Bsully ... if your canopy is open back, do like the one poster suggested and get a fan or fans to blow in there. I would set it up so you can kind of circulate the air blowing in on one end on an angle so it can flow back out on the other end of the opening of the canopy. If it is closed back, you need to cut some vent holes and install fans. If the canopy is lower than 6" at a minium, I would not use the halide in there. You probably wont get good enough light spread anyway.