For this project you will need some mortar, a length of dowel between 9mm and 11mm thick, and some lengths of pine. I used 1x4s (90mm x 19mm) as they were cheap and easy to find. You'll also need some copper pipe and copper pipe fittings.
Start by cutting the pine into pieces. I created one piece that was 90mm x 60mm. The rest of the pieces (4 of them) were all 90mm x 70mm. Keeping one side 90mm means you only need to cut the boards along the length. This can be done easily with a mitre saw, but you could also use a hand saw.
Once you have all the pieces cut drill holes into the bottom piece. In my case this was the 90mm x 60mm piece. I drilled 4 holes.
Now you can create the box. Drill pilot holes and screw the 4 sides into the bottom piece. This doesn't have to be too pretty, as long as the joins are tight. You can see how I created the box in the picture.
Put the box aside and cut small pieces of copper tube using a pipe cutter. This is 12mm copper pipe (1/2").
Take some t-joints and push dowels into the side openings. You just need to plug the holes so even if it's not dowels that's fine, just plug them so mortar doesn't run into them later.
The top part of the t-joint is what the pencil will go into so the pipe should be cut to length accordingly. Make sure you have around 10mm - 15mm of mortar around the copper to ensure it doesn't crack.
Now you can dismantle the box and apply packing tape around the inside. The mortar won't stick to the tape so it's good for releasing from the mould. You could also apply some oil to the inside to help it release better.
Once the box has be reassembled insert the pipe fittings. Make sure they are oriented so that they have at least 10mm to 15mm mortar around them otherwise the mortar might crack. Your spacing of the holes will determine the orientation.
Mix up some mortar and pour it into the mould. I added some coloured oxide powder to mine to give it that dark grey look.
Once the mould is full tap the sides with a hammer. This will remove the air bubbles and make the mortar set stronger.
Wait 24 hours then remove the sides of the mould. At this point wait another 2 days for the mortar to dry further.
Once it's completely dry to the touch remove the bottom piece and the wood should slide right out of the copper pipe.
To seal the mortar you can use a water-based concrete sealer (as I've done) or something similar. This makes sure that you don't end up with cement dust everywhere.