I went on to the popular auction site (you know the one) and got these clear pencil cases for 99p each. I measured the pens first (15cm) and found that there were a lot of 16cm and 17cm high pencil cases available. Factoring in the thickness of the pens I opted for the 17 cm size (23cm wide), and 15 promarkers fit snugly inside. Of course all I have to do now is to figure out what order makes for the best arrangement!
Oh, I know that in the pic it looks like these cases are clear on the front and brown on the back, but they're actually clear on both sides, it's my brown floor showing through from behind! :o)