Someone recently shared this video (in a comment on my Super Simple Passive Worm Farm) and I thought it was really interesting.
A variation of this bed could be used as a great winter system, but I would recommend filling the air spaces with something (I dunno – sawdust?), or – even better – using straw bales for the walls. Also, I would recommend digging down at least a foot or two. This could be a helpful strategy in very hot locations as well.
If you have really heavy clay soil you may also want to create a drainage channel to remove excess liquid (from rain, snow melt run-off etc). Instead of the screen, a couple of layers of pond liner felt contoured to the shape of the depression (again assuming you are digging down) and secured under the walls, could work well.
NOTE: The average number of babies per cocoon for Red Worms is actually 3 (not 4-20) – to learn more about Red Worm growth rates be sure to check out this post: Will A Red Worm Population Double in 3 Months?“