The first guns
shipped in featureless were Robinson M96s that looked like this, just taking the pistol grip off:

A lot of people worked on mods to make existing guns legal with funky stock and grip alternatives:
The U-15 which is designed to also NOT be a thumbhole stock (called out in the provisions explicitly)

The monsterman grip which only really works right with A2 stocks.

The Hammerhead which post-dates the mostly defunct U-15, but cheaper and more customizable since it's just a plug that replaces the grip which buffer tubes and stocks thread into. This results in even stupider looking firearms but it's far more versatile.

Apparently Mr Fantastic Owns the below specimen, that's an A1 length Ace stock tacked to the back.

Half of those are from NY, all pictured were compliant firearms at the time of their photographing.

lol
Interestingly there have been some more serious attempts to make something that doesn't look completely stupid at significant commercial risk:
A redesign of the lower to use a more shotgun like buffer system, I know some that look very much like this have hit the market in limited numbers.

And uh...whatever this cross between an R-18 e-tron and an AR-15 is...again with a lower redesign, but no buffer redesign. I know this one never really hit market unless something changed.
That said, being able to wrap a grip, any grip, in Kydex and modify nothing else except yanking naughty bits like flash suppressors or fixing adjustable stocks with roll-pins is far easier than any of those prior solutions for the most number of people so most of these other solutions fell to the wayside.