Redditor Halvors7 has published the printable parts to build his FN P90-inspired Charon AR-15 lower. The Charon features an integrated buttstock, handgrip and trigger guard. The stock is fixed but can be adjusted for length-of-pull by using spacers. The lower even supports a number of P90 style foregrips!
The printer files can be downloaded at Reddit. If you want to support the development of this stock, the Halvors7 is accepting anonymous donations on behalf on the creator, WarFairy, via. Bitcoin (1JwXiC883Rh3GQ7jTWPzLAEEterL29Z2n2).