This project is going to make your gardening more of a hobby and less of a chore!
Simply buy a piece of plywood--any size will do--and measure out a grid of dots to represent the holes you want.
Drill a small scree through the dots.
Apply some permanent glue to the exposed screws and screw on corks, which you can buy in bags online or at hobby and craft stores.
When you're ready to plant seeds, pack the dirt and place the board--cork side down--into the ground.