From: Alameda, CA
There are no polygons for hexside terrain, those are always just the single line on the hex border for LOS. To make panels, save it normally as .map and then rename the file to .mpp in windows. And this has to be a single map panel. That should work, I tested it in 1.08 by fussing with the default map, saving it, renaming it to .mpp and loading it as a panel on some other maps.
But if you're making large multi-panel maps there's no reason not to just leave it as .map and delete all the units to reuse it.
The only snow rule I noted down while looking at the board game scenarios was Fresh Snow, changes Clear & Road to cost 2 MV for SMC/MMC. There might be some others hiding somewhere, of course - I looked through Day of Heroes, Blitzkrieg, Swift & Bold, and Not One Step Back.