I just picked up an 11721 yesterday and have been on here looking around also...
It seems that GG&G has an option: https://www.gggaz.com/benelli-m4-accessories.html
IWC has a couple of options: https://www.impactweaponscomponents.com/myshop/product/view/my/category/benelli-m4/
OR you can change out the foregrip for a rail system...it appears there are a few choices folks like:
Agency Arms - appears very difficult to source
Briley - https://www.briley.com/p-63643-briley-3gun-m-lok-handguard-benelli-m4.aspx
Midwest Industries: https://www.midwestindustriesinc.com/product-p/mi-benm4-hg.htm
There are a few other models but appear to be not as well liked as the above...once you go with the m-lok setup there are a couple of options:
Arisaka: https://arisakadefense.com/inline-scout-mount-m-lok/
T-Rex: https://www.trex-arms.com/store/Lightbar-Scout-Mount/
That is all I have found but I just joined this forum today and have only got about an hour or so digging into it...maybe a more experienced member will fix what I have broken!