I have two sizes of seed presses.  They are custom made to fit inside the open seed trays I use in the greenhouse.  You can make yours any size you would like them to be.  A good starting size is 12" long.

What you will need:

• A short piece of 2x4 wood of the length you want it, or whatever the length happens to be.
• A fairly large handle with screws.  You will find a large smooth rounded metal handle is going to give the best long term satisfaction.  Mine is a hollow aluminum, smoothly rounded and long and high for easy finger grasping even with gloves on.  It's called a gate handle.
• A screwdriver to attach the handles with the screws.  Screws will stay in the wood much longer than nails will, which is why I suggest them.

Center the handle on one 4" side of the wood and screw it into place.  Your done!