groundFungus:
Could you protect them with a movable cover? Have a small servo to open the cover long enough to get a range and then close it.
Unfortunately, that's a direction I do not really want to head for, because:
(1) It's currently an extremely energy-efficient device that can run off of a single D-cell for 4-5 years. A servo would drastically reduce this lifespan. I also plan to get more of these devices made and the logistics of battery replacements would become tiresome (compared to a quick wipe of the sensor surface)
(2) It's meant to be IP67-rated, and it will become really tricky designing an IP67-product with moving parts at an affordable cost.
I would probably favour cleaning the sensors every now and then over dealing with servo motors in these things, but it's a pretty interesting idea for sure
INTP:
I wonder what kind of garbage is so effusive that it slathers up a sensor from underneath.
Yes, what @wvmarle said. Waste bins are different throughout the world, but the dumpster ones in North America get lifted and tilted above the truck, so that everything falls out of the top of the dumpster. Any sensor therefore takes a pretty big hit every time, in terms of liquids, grime, organic products etc. Some of the worst sensors (not ones I made) I've seen had been on dumpsters for 1-2 years and were coated in a cake of 1-inch thick organic waste.