When I started growing tomatoes, I transplanted the seedlings into standard 6-pack cells when they got their first true leaves. This worked ok if they didn't stay there too long, but when I found that the plants did better if I waited an extra week after the last frost date to set them out, I found that they tended to become root-bound in the small containers.
I tried several alternatives. Paper cups (8 oz with holes punched in the bottom) gave the roots more room, but were a little wobbly and the bottoms tended to rot out when standing in a tray for bottom feeding. Some 2x2x6-inch hard plastic cells that held mail-order plants had plenty of room for roots, but it was difficult to extract the plants. Ronaash Root-trainers produced excellent root systems, and it was fun to open them and see how the roots developed, but they were spaced so close to each other that when the tomato plants reached a foot tall, their tops became so tangled that I sometimes broke the stems trying to separate them.
One day I was opening a 10-oz can of frozen orange juice, and happened to think that the empty container was just about the right size for a tomato seedling, so I started saving them, and this year when tomato planting time came, I had about 20 of the OJ containers. I punched holes in the metal bottom with an old ice pick, filled them with a good moistened grow mix, and transplanted the young seedlings into them, burying the stems as deep as possible. I put them into two trays, 10 apiece so as not to crowd the plants, and watered them from the bottom. When they got too big for my light frame, I moved them outside into a plastic pop-up planthouse.
A flurry of events caused me to postpone transplanting the tomatoes in the garden until May was half over and some of the tomato plants had reached 18 inches high. Nevertheless, they were all growing well, the sturdy OJ containers held them upright (only one flopped over), and when I set them out, by pulling at the spiral seam on the cardboard sides, the containers unpeeled easily, revealing a good deep root system which was better developed than the extra plants I plunked into 6-packs when I ran out of OJ containers.
If you are an orange juice drinker, I recommend that you try this yourself. I have already started saving the containers for next year. One oddity was that many of the cans developed tiny mushroom growths at the bottom rim where they sat in the watering tray. It didn't seem to affect the tomatoes. Sorry I didn't take a picture, but I was in a rush to get them set out and didn't think of it.