NavList:

You wrote:
"In the summer I have heated the glass plates before using it and then they are clear for several minutes."

This is certainly the way to go if you can keep it up. You can heat your glass plates continuously. Run an extension cord to your observing spot and bring a handheld blow dryer. Hit the glass with hot air just before each observing run. That may be all it takes.

Backyard astronomers have lots of tools for this sort of thing though mostly adapted for circular telescope tubes. Also in astronomy, the problem is moisture from the air rather than moisture from a reservoir. Nonetheless, you may get some useful ideas from this article: https://skyandtelescope.org/astronomy-equipment/equipment-diy/dealing-with-dew/ published in 2006. There are some good ideas for homebuilt heaters here: http://www.deepskywatch.com/Articles/newtonian-dew-heater.html.

The suggestions from Greg Rudzinski, Ed Popko, and Brian Villmoare are also excellent. Brian's recommendation of Rain-X and related products is a new outside-the-box idea. In fact, in the world of the celestial navigation enthusiast, outside-the-box often aligns with COTS: commercial-off-the-shelf. Our problems are not unique. Of course there are commercial products that prevent fogging! They sell it at Walmart. :)

Frank Reed