Hi...
just a suggestion...instead of using heavy sheet metal, try using heavy aluminum foil...its alot lighter...linda

A definite possibility, though I already have the sheet metal, so as long as I'm not making things "worse", I'll most likely just stick with that.

I think a reflector would be the best thing you could add. I would line the sheet metal with aluminum foil unless it's already polished to a shine. Dull sheet metal might make a worse reflector than the white fixture itself.

As for the heat, you can judge it with your hand after it's turned on. When I ran a 250W MH, I kept it at least a foot from my plants. Any closer than that, and I could feel the heat with my hand. A fan blowing under the light may help with this as well.

Is your bulb an Agrosun and, if not, what brand and model is it?

This is the spectrum of Agrosun MH:

This is the spectrum of Eye Hortilux Blue MH:

This is the spectrum of a regular MH, Sylvania Metal Arc:

You can buy regular MH bulbs for as little as $10 here.

Notice that compared to Eye Hortilux Blue, the other two MH are low in far red above 700 nm. I can tell you from experience that adding incandescent to Sylvania Metal Arc, greatly improves growth, presumably because of increased far red light.

For either 250 watt Agrosun or regular MH, I suggest that you test adding a 100 watt incandescent bulb.

Here is the spectral density chart of Philips Ceramic Metal Halide which many other people claim is very good:

Thanks - I'm not sure what type of bulb I have, and am sure I need another one. I have three T-8 fixtures (two 2' and one 18") that I'm planning on using for supplemental if needed, though I'm hesitant to buy anything more than hanging accessories, heavy duty tin foil (for a reflector), and an emergency blanket (mylar). This is still just lettuce, and proof of concept for the next ~45 days. If things go well, I might be able to take my little 18"x32" plot and double it's size.

I am slightly nervous about the heat coming off this lamp from a "it's near a wooden shelf and how far away from the wood should I have it?" perspective. Again, with a 250 Watt MH, what is your recommendation? I have fans that can blow on both the plants and the light.

Isn't the brand and model written on the bulb?

I suggest you keep HID and incandescent bulbs away from combustibles. The outside surface of some halogen incandescent bulbs is reputedly hot enough to start a fire.