As a fan of Lego and firearms I am not sure if it is a good idea to make a hybrid of both. Alloutdoor posted this the other day.
Feinstein Gewehr Werke, LLC, is a plastic brick kit manufacturer. They make the Mk8 stock for milspec buffer tubes. The kit costs $40 plus s/h and tax. They posted free instructions online to build the stock. The stock is not just lego bricks attached to each other by their clutch power. Clutch power is the ability of its bricks to snap together tightly while also being easy to separate, thereby readily allowing for de- and reconstruction. Feinstein Gewehr Werke uses super glue, epoxy and steel rods for support.
The multi color nature of the stock is due to costs. You could look at the instructions and figure out the parts required and order them through online resellers of individual bricks, like Bricklink.com
According to the website, the makers of the stock tested the Mk8 on a Remington 870 12 ga with 24 rounds of 3″ magnum 00 buck.
Here is a link to their video.
The design of this stock and the lack of any recoil pad makes it look extremely uncomfortable to shoot. Especially with a shotgun.
They are also working on something they call “The Feinstein Project“. Building an AR lower out of Lego bricks. If you click on their “Lines of Effort” you can see some baby step projects that could lead to a functioning lower.