...just got hold of this lamp: has a built in ballast, a 250w hps bulb and is small/light enough to hang from the ceiling. I want to make some kind of reflector - could it be a cardboard funnel (see my poor diagram) covered in tin foil or will this burn? - what about alu sheeting or somethin similar? Any suggestions?
I guess its coated with something. If I was you I would definately get myself a cheap reflector made out of ALUMINUM!!!! You don't want to start a fire,especially if that is a HPSodium it will give out a lot of heat!!! Take my advice and use a metal reflector,no matter if you buy or make one. Do it right man!!!
don't use cardboard or you will burn your house down. If it doesn't have to support weight, make your reflector out of a roll of aluminum foil. Just unroll a bunch of foil and fold it back against itself till you have a piece a couple feet long and 10 or so layers thick, that will work and be the cheapest$ way to go.
Way back in the days when cash wasn't about i used a very weak chicken wire to mould a shade, which i then covered in tin foil, this worked ok, but the reflection values are pretty poor, just because most shades are tin and shiny doesnt mean foil is as good as your brain thinks it is..lol. For best reflection (matt white).