Please note that when we speak of "reflectors", we aren't talking about the entire fixture, just the part of the fixture that reflects light and nothing more. Some fixtures have large reflectors, some have none, most have something of a reflector. In all cases, these reflectors are useless for reflecting UV. They only reflect visible light.
The majority of our UV lamps for UV curing and horticulture are RUVA style, meaning they have a built in reflector which is more efficient than any standard material available. This means they only shine light in one direction, towards the item being cured/grown, and never behind the lamp. In most cases, this is adequate and you will not need any other reflective material. This is one of the advantages of using Solacure lamps: they are simpler to work with and save you time and money because you just install them and go. There are some applications where reflective material WILL help you. These are situations like guitars or pool cues, where you are exposing four sides of an item at the same time. Adding simple reflective material in those situations can bump up the UV around 10% or so. Not dramatic, but not trivial either.
If you must use a reflective material, the absolute best is also the absolute cheapest: aluminum foil. The shiny side is obviously best, and it is best if you crinkle it up just a bit when installing, so the reflections are in multiple directions, and you aren't creating hot spots. In your UV rig, this can be done by spraying a thin coat of spray glue (3M makes a really good one that I've also used for recovering pool tables) on the backing, then press the aluminum onto the glue. It doesn't need to be perfect. For bigger rigs like those used on violins and guitars, you need to use it on the top and bottom, so the butt end and headstock will get UV as well.
Photo courtesy of Kmashaye5220 @ Wikipedia Commons
As you can see in this example, it breaks light up nicely, plus you can clean it with Windex or any glass cleaner. Again, the lamps won't create any light behind them, this is only to catch light from lamps across the item. You can get creative with this, mounting on cardboard for the upper and lower sections, plus you can easily patch or repair this. Most importantly, aluminum is about 50% efficient in reflecting UV, which doesn't sound like much but most paints (like the paints used on generic light fixture reflectors) are less than 5% efficient, so this is miles better than any other off the shelf product. And it is dirt cheap, which is also nice.