I got this case and it works great for holding the Pixelbook and the pen. I might get the next size up though, it's pretty snug.

Tomtoc Ultra Slim 13 - 13.3 Inch MacBook Air | MacBook Pro Retina 2012-2015 | 12.9" iPad Pro | Surface Laptop Sleeve [Felt & PU Leather] Bag with Accessory Pocket, Support up to 12.8 x 8.94 In https://www.amazon.com/dp/B01HCV22IM?ref=yo_pop_ma_swf