Hi. Welcome to my instructable.
Do you want to hide your secrets or you want to be a bit more private?
So, I'll tell you how to make one for yourself with very simple things. Let's get to work.

Supplies

Wooden blocks, Adhesive Glue, Scrap papers, Lemon Juice, Old Fountain Pen

Step 1: Get It Sticking.

Arrange the wooden blocks according to the dimension of the virtual notepad in your mind(as you want it) :)
Slice them off if you need it to be slim.
Apply some glue and stick the wooden blocks together.
Let it dry.

Step 2: Making I T Look Better.

Polish or paint the newly made pad the way you like. Stick 2-3 papers one over the other to make the surface smoother.

Step 3: Writing on It....

Clean your fountain pen. Extract some lemon juice from a lemon and fill it in the pen. Make sure to sieve the juice(you can also use a cut-nib pen or calligraphy pen). Start penning your thoughts and secrets onto the notepad. It would be a bit tough, as you could little see what you are writing.

Step 4: Reading Your Secrets.

You could see it as a blank paper after it dries. So if you wanna read it, just run a hot iron over it. The heat burns the decomposed paper (only what you have written) and creates clear text. You can be pro in no time, it's very simple and out of the box!
Thankyou!