What you need:
Five 2x4s, any lenght you wish, as long as theyre all the same.
Two metal corners the lenght of the 2x4s. (Look at the pictures)
Assembly
Put two 2x4s side to side and screw them together. Repeat this with two other 2x4s. Screw diagonaly the two pairs of 2x4s one over the other. Cut in some stabilizers in the remaining 2x4. Use your judgement for the lenght. Screw them perpendicularly under the 2x4s. Once this is done, take your two metal corners and place them as seen on the pictures. Drill some holes in them (only on the side so your board doesnt get stuck while sliding) and screw them to the wood.
Last touch
You can leave it like that but it looks better if its painted. Just use your imagination.
