Ultrasonic invisible material?

Hello all,
I recently started a home project in which I am using an ultrasonic range finder to measure the amount of oil in my oil tank. While building the housing for it, it occurred to me that putting electronics near flammable liquid might not be the best idea, so I was wondering if any of you guys knew of any material that would allow the sensor to function but also remain internal to the housing, or any other ideas.
Currently my housing consists of a PVC adapter (so I can screw this thing into the tank) with plexiglass on top to seal the fumes in and a small hole with a rubber grommet for the wires. The sensor fits inside the threaded portion nicely when drilled out appropriately. I am toying with the idea of cutting another piece of plexiglass inside the adapter and drilling a couple holes for the ping and receive (this is the sensor http://www.radioshack.com/product/index.jsp?productId=23173056).
Any ideas or critique etc would be appreciated.
Thanks!

Oil for heating is not very flammable. Coworker tells story of furnace repairman throwing cigarette in bucket of oil. Result? Cigarette went out.
Ultrasonic is just sound too, not even radio waves.

Have you considerd ths

Thanks CrossRoads, I may continue on this road then. Grumpy_Mike, I did not consider that but as it is 10$ more and exactly what I wanted I may pick that up!

Oil for heating is not very flammable

Well it must be, else you'd have a cold(er) house. What I read, it's pretty chilly in the Northern Hemisphere right now.

It's the flashpoint that's important here I think, not sure of the terminology. Even petrol / gasoline apparently won't light from a cigarette, so those movies all wrong 8)

I'd ask my son- he's a FF/EMT but he's got a 12h medic shift today so he's catching a few last zzzzzzzzzzzzzzzzzzzzzzzzz's.