You can use egg shells and the cartons they come in. Of course, it might take you some time to get that many shells to start 400 plants. Take a look at the link....hope it helps.
Burpees seed catalog has a little pedestal type thing you can buy that will press containers out of newspaper. You then fill these wei soil and stack them in nod degradeable crate or something to hold the shape of the whole. Not too expensive and might possibly be easily copied if you are a wood worker.