Saw this idea a long long time ago on the internet somewhere and now have finally made it.
It's just a box, covered in tape for strength. The foam alphabet stickers are a little tempting to the kids to pull off, but I've managed to keep them safe thus far. I wrote the matching letters on paddle pop sticks.
It's a fine motor activity and a letter recognition challenge! Hugo just likes to put the paddle pop sticks in, while Sophie enjoys the challenge of matching things up. If I want to make it more difficult later on, I can write the lower case letters on the paddle pop sticks, to match with the upper case foam stickers.