The beans I recently started from seeds for my fall crops are growing like mad and will need to be bumped up before I transplant them in the garden. To allow room for them to grow when we bump them up (transplant) we built a couple more seedling flats.
In your Mittleider Gardening Course book you can find a description and dimensions of the seedling flats on page 181.
The flat was filled with the same sawdust, sand and perlite mix that was used in the first one where our seeds are now growing. The beans will be transplanted into the new flat soon.