I'll be putting these up for sale soon, I designed a spacer to go under the OEM Comb on the collapsible stock, can raise it in 10, 20, or 30mm intervals. Includes all bolts needed for the different heights. I prefer the EOtech Holo donut of death, so I made a product to make it usable on the C-Stock for a good cheek weld. It is rock solid in all heights, the bolts are more than adequate to resist lateral force from the face.
You can check it out at https://rxarms.com/products/benelli-m4-parts-and-accessories/benelli-m4-cheek-riser/