Based on the great ideas in this thread, I have started my passive seed starting experiments by winter sowing!
There have been multiple snowstorms here and even with a bit of melting the snow still is approaching knee-height, so I haven't been able to physically get into the greenhouse where a bunch of old lights and seed starting trays are. I'm sure I will be able to get in by April, when I really need the equipment for some TPS I'm trying to grow this year (although now I'm wondering if I should try winter sowing some TPS since they love the sun so much). So for now, everything will be winter sown or direct sown.
Turns out my parents go through a LOT of distilled water with their CPAP machines and humidifier so they came and delivered a giant stash of bottles perfect for winter sowing tomatoes and other big ol' hot weather plants. But in the meantime, I've been saving and using old plastic food storage containers that got cracks in the bottom (volunteer drainage holes), old clear produce boxes (some with bottoms doubled up to make larger greenhouses, yogurt cups with windows cut into the lids using plastic produce bags (the kind with holes already in them), and whatever else that can be made greenhouse-y. Labels were done with some oil-based markers that were gifted to me and cuts sealed up with some gaffer tape. Holes were drilled or, if the plastic was more finicky, a hooked sharp knife worked perfectly for making holes or 'x's.
I also took Dr. Redhawk's suggestion of reactivating some old potting soil that was left on the property with a mushroom slurry and homegrown Lactobacillus for the planting. Mushrooms were collected from some punky firewood and some leftovers from the fridge, the milk used was from making a quiche.
So far seed starting materials this year has cost me $0 (except for a couple new packets of seeds.... I couldn't resist). Sharing photos below of my winter sowing adventure, I'll continue to update my passive seed starting journey as I go along.