This was a neat project I completed last year while on maternity leave. My daughter was learning to pull herself up at the time, and loved grabbing things. She also loved pulling on cords and trying to destroy my computer. Killing two birds with one project I created a Mondrian inspired activity board for her so I wouldn't have a heart attack every time she crawled under the desk. It also has an attachment on one side painted to look like a 1980s New Zealand Post box. Attaching to the underside of the computer desk, it successfully hid all the cords and plugs and kept her entertained for ages - which for a baby is measured in 10 minute stretches ;)

The materials are relatively inexpensive if you're lucky and catch a sale of odds and ends at the hardware store and have some old poster paint in a cupboard:

Any kind of robust board/wood/MDF cut to fit across the back of your desk.
Small angle brackets and/or robust zip-ties to secure board under desk and to the back legs.
A selection of knobs, handles, rope stretches, castor wheels, dials, switches, things from your garage with interesting textures or shapes - anything shiny or textured really.
Wood offcuts or similar (not essential, but some of my handles needed a thicker backing to screw into.
A selection of screws or bolts if none came with your handles and knobs e.t.c.
A selection of poster paints (for a Mondrian board these should be black, white, blue, red and yellow).
A role of Moshi tape - Black if you're doing Mondrian. (Moshi tape is more commonly used in scrap booking I think but it was brilliant for this.

Step 1: Fitting Base Board

Measure and cut your board so it fits under the desk and reaches to both back legs. Make sure it doesn't sit too tightly into the carpet at the base (otherwise the desk is hard to move), or too loosely (otherwise little fingers might get stuck underneath).
Work out how you're going to attach it to the desk. I used very small angle brackets to anchor the top to the underside of the desk and ties/cable ties through drilled holes around the legs.

Step 2: Layout

Place out the pieces you've got:

see what looks visually interesting
Consider what parts your baby is likely to like the most and put them an enticing distance away from where they can currently reach.
Start to think where the biggest Mondrian squares of colour might go and what piece you'll put there
Move them around 'till you're happy, then mark your decisions in pencil directly on the board as it will be covered up in paint later.

Step 3: Mondrian Painting

I drew out my squares and oblongs based roughly on a Google image search result for 'Mondrian' then adapted the layout to suit my bits and bobs.

Paint the squares - don't worry about the edges. As long as you don't go over the lines too much, the Moshi tape will create nice black edges after the paint has dried.

I used poster paints. I didn't have any fixative at the time so a few dribbles from the babykins have caused a little paint bleed over the last year, but nothing drastic. If you don't want paint run - spray it with something or use different paints.

(Note - you'll see from some of my pictures that I laid out a track that doesn't feature in the final version. I didn't own many power tools at the time but I tried to build a track for a handle to be moved through instead of routing a channel for it. It all worked on paper but my final version was rubbish so it was removed and painted over.)

Step 4: Moshi Taping and Fixing All the Bits

Moshi tape is a beautiful thing!

Start with the shortest edges you have, making nice clean edge cuts.

Once you've done those, do nice big stretches down the longest lines you have until all the squares and oblongs have nice, clear black borders.

Now you can begin drilling and screwing on each of the things you've chosen. I found the furniture handles the most troublesome as I had to glue thicker wood offcuts to the back in order to fix them stoutly. Rope is the easiest requiring only a good knot tied in one end and the other fed through an appropriate drill hole.

Step 5: Optional Extra - the Post Box

One of the pieces I got on sale from the hardware store was a piece of metal that could be fitted inside an oblong hole creating a posting slot. That got me thinking about mail soon being obsolete, but what a fun activity to share with my baby! In the 1980s, New Zealand postboxes were an iconic black red and white design so I decided to add this as a side piece to the Mondrian board.

Pieces:

From offcuts, I had a tall, thin oblong for the box - as tall as the board itself and wide enough to look right (I just judged it by eye on the day I was cutting it).

I cut the hole for the post slot (using a jigsaw - it was just precise enough to cut a basic hole), then measured out by eye roughly where I wanted each of the three coloured bands to begin and end.

In the main black base area, I cut a square for retrieving the posted mail.

I also cut two small strips from offcuts to glue to the top and bottom to evoke a top and base to the box

From an off-cut, I cut another square out, one that overlapped the retrieving hole all around by about a centimetre.

Paint:

Firstly, I painted the whole of the piece in white as a kind of undercoat.

Then, using leftover Moshi tape, I masked off the an area to paint red in the centre.

Once dry, I moved the moshi tape (as it peels easily) to mak off the bottom red edge, then painted the whole of the bottom piece in black. Once dry, the Moshi tape removes easily leaving nice clean lines between the white, red and black sections.

Logo:

I took a photo of a current post box so I could crop the image of the current logo for NZ Post (It's still very similar to the 1980s one) and the collection times label.

I imported the photo into MS Word and played about with the image size and cropping until I felt I had the right size.

I colour printed this, cut the pieces out, then glued it to the finished box front using a spray can adhesive I found at work.

The completed box was my daughters favourite thing about the whole project!