I like the idea of NOT throwing away things that can't be readily recycled -- i.e. that my limited municipal recycling won't take. At least not right away.
I'm basing the depth of container on J. Jeavon's 3" deep for sowing seeds and 6" deep for most uppots. (Lettuce can stay in 3" deep). Filled with soil to the rim for better air circulation.
So I've been making seed flats out of milk and OJ cartons and ultra-pasteurized Rice Milk cartons, as well as corrugated cardboard boxes (well they take that, but I'm not giving away CCBs when they can be put in my compost pile afterwards.)
I found out that it's easier to cut open the Milk, OJ, and RM carton flats when it's time to uppot. Then the seedlings can be easily eased out.
For uppotting the seedlings, I'm finding that typical nursery pots that are cubular (if that's a word) in shape -- same height as width -- are shallower than I want. I'm liking the various sized drinking cups, yogurt containers (NOT Yoplait because they're upside down , and quart size deli containers because of the extra depth for deeper roots. The narrower shape saves space too.
Of course I have a lot of nursery containers from previous purchases as well. I like the square ones better because they hold more than round ones for the same width/diameter. I'm also experimenting with Chinese food pt and qt size paper containers. I use the deeper than wide (is there a geometry word for 3D rectangle shape?) containers for uppots and cubular ones for community seed starts. Depending on the kind of plant, I can pot them up in a group of 2 or 4. 2 on opposite corners or 4 in each corner, planting closer to the edge to minimize root entangling.
I'm also using 6" deep CCB's but it's harder to find drip trays for them. (Aluminum lasagna pan works well)
Plastic take out trays are useful as drip trays for smaller containers, and for elevating seedlings up to the light. Their lids are useful for that tiny bit more height, if you're micro-managing.
All these smaller containers provide a lot of flexibility when it comes to moving the seedlings about -- from the heating mat to the lights and shuffling around under the lights and eventually for hardening off.
Flip side is there is a fair amount of wasted space no matter how carefully you arrange them, but then again, this might not be entirely a bad thing because there is more air circulation between the containers and the plants.
What are your ideas?