I've found myself using this icon a lot since the first episode of the new series aired:
So I figured it was time to make it more Who relevant:
It's not perfect, but I think it captures the spirit of the original. I hope.
With profuse apologies to Antoinette Portis who wrote the original picture book about a bunny and all the things his "not a box" can be (and judging by the adventures it has, it may well be a TARDIS). Which is lovely and one of my favourites and you should all run out and buy it.
ETA: feel free to take with credit