First off - thanks to everyone for their suggestions. They were helpful. I decided that the best thing to do was to give up my attempts to *make* something when I saw this -
Combined with a couple of 1" self-stick reflectors and some LEDs, and I've got my level indicator. I mounted the sensors and the reflectors on a pair of 1" (wide) pieces of aluminum so I could align them exactly. I tested them out and, although some material does stick, the sensors see through it just fine. Sometimes it's better to buy rather than build; the key is knowing when to abandon your own idea for someone else's.