The DiveRite wetnotes are great and at $15 the price is right, too.
However if you want to go the cheap route go buy a small piece of white linoleum (sp?). Drill 3 holes in it--one in a corner and two at the opposite end towards the middle , about 1/2" apart.
Sand the piece of linoleum until you can't see ANY shiny part. This might take a while but it's the part that lets you write on the board. Sand down the edges so they aren't sharp.
Take a small thin piece of bungee and put it through the two smaller holes. Take a small golf pencil and put it under the bungee--this is what will hold the pencil. While there's tension on the bungee turn the slate over and make one small overhand knot. While someone is holding this take a very small zip tie and zip tie that knot! NOW take superglue and glue all that to the slate. This is a secure method for holding your pencil.
The other larger hold in the opposite side's corner is for another zip tie, which holds on a small clip so that you can attach it to your BC or whatever if you wish.
Buy a Suunto A10 flat compass and put it in opposite corner from the clip, pointing longways down the slate. Superglue this compass down, but DO NOT glue the rotating bezel. Obviously this needs to move so that you can properly navigate.
This slate will be easy to write on with a small golf pencil and can be erased as well. If you want to get it super clean after use just sand it a bit--especially if you decided to use permanent ink. (I have to use permanent marker to put a required skills list on the front and a map on the back when I'm with new students)
If I can I'll post a pic...