I did it, I finished my little Christmas project! I actually wasn't sure whether I'd get it finished for the blog post today as it took a lot longer to make than I first thought...
Here's my finished advent calendar using a cheap baking tray and magnets. The principle for this is that you remove all the magnets (all the little date squares) and store them on the back of the sheet. Then on the 1st of December you add the first square to the grid and so on, until you have added them all and it's Christmas.
The tray calendars that I have seen so far have all been based on a 5x5 grid and counting down to the 25th of December. This didn't really work for me as I celebrate Christmas on the 24th of December. Time to think. After a while I came up with the plan of making the 24th square double the size. Just like the bought cardboard calendars that you can get in the shops, that quite often have a bigger opening for the 24th.
I used the "Christmas Dreams" and "Jolly Lil Friends" kits by Mindy Terasawa at Designer Digitalis to create it.
I'm not sure how well it shows up in the photos but there's lots of yummy Stickles glitter involved etc :)