I had seen a similar thing by Askyra (see "remixed from"), but probably because of issues with my tiny printer, it didn't fit properly. While creating my own I also decided to make minor changes.
These are made to mount a sacrificial bed, but can of course also be used to clamp down other material directly. Main advantages are that you don't limit your workable area and collisions between holder and spindle are less likely.
There are three sizes: short (15 mm), medium (25 mm) and long (30 mm). I created the short version first, but I wanted to add more space between tool and holder. The long version allows me to use wing nuts to hold the bed down and avoid contact between the wing nuts and the spindle
You can then use bigger beds with space outside of the workable area for clamps or screws.
Right now I am using 340 x 240 x 12 mm MDF boards. (Tip: drilling holes in MDF
will make the bottom tear out. To avoid that, support the board with some scrap wood or similar.)
The holders are made for M6 hex head bolts that came with my 3018 Pro clone and the corresponding wing nuts. Especially when working with a 3018 Pro, make sure to push the M6 bolts all the way in their spot so they don't crash into the bottom plates.
Please let me know in the comments if and how these work for you or what you'd like to change / improve.