while running a search to find a link for a different thread, i stumbled across the fact that someone is actually producing these things. there was a calgunner who designed one a while back (see here), but i don't know if this is him or someone different.
three different models with varying lengths and angles; each is $45. according to the site, they fit mossberg 500s, 590s, 590A1s and maverick 88s.

three different models with varying lengths and angles; each is $45. according to the site, they fit mossberg 500s, 590s, 590A1s and maverick 88s.

