I mostly use "standard" seed trays.

These are 23cm x 37cm and have many drainage holes in their bottoms.

The seed trays are placed inside "standard" gravel trays.

The gravel trays act to catch excess moisture - and to channel it into a single drainage hole.

This shows the arrangement I use when fitting three seed trays into a gravel tray.

The seed trays here here were originally containers soft fruit were sold in.

This shows the arrangement I use when fitting four seed trays into a gravel tray.

The seed trays here are commercially-available seed tray inserts.

This shows the arrangement I use when fitting five seed trays into a gravel tray.

Again, the seed trays are commercial seed tray inserts.