The Bigbox heated bed is a 300x200 bed backed with cork and is supported at each corner. Alas, it sags a small amount toward the center. These are designed to provide a little support at three positions underneath the center of the bed.
Fully closed, the bolt will be 22mm. The bed itself is around 25mm distant from the platform below, but could be slightly lower or higher as a result of levelling using the corner spacers. Unscrew this slightly to fit your needs; it should happily support as low as 22mm or as high as 28mm.
I chose a 12% rectilinear grid for the fill, and 3 outlines to give strength to the rod while twisting. I don't recommend printing at 100% as this will create a lot of strain in the part as it's made, probably leading to warping. This is not a heavy duty requirement, we're just pushing the bottom of the heated bed up to the glass in the center.
You could print at a finer resolution, but this is a hidden part. Your choice.