The general design of 2-season tents is that the mesh goes almost to the floor of the tent, with a rainfly that's held away from the mesh by a few inches, so there is lots of room for air to move through.

On a winter tent, the idea is to let much less air move through, but be able to have some air movement, to prevent condensation and frost. So, to best make your 2 season into a 4 season, you want to cut down on the air movement. To me, the simplest way to do this seems like cutting up that 6 mil poly so it fits over the large mesh openings, and turn them into small mesh openings. You could tape the plastic over the mesh, so it blocks most of the opening, but still leaves some space for air movement. You will likely have to play with this a bit, to find a good balance between keeping your warmed up air trapped in the tent, and letting enough air circulate to prevent condensation.


Another hint is to use a chimney effect to help air circulate - a small vent near the bottom of the tent, and another near the top, will allow warm, moist air to move out of the top, while cool air is drawn in through the bottom. You obviously don't want a lot of cool air to come in, but you want to let enough of the moist air out to prevent getting soaking wet from condensation. If you wake up in the middle of the night with a lot of condensation, peel away some of the plastic, to let more air flow, and that should help.


Good luck!