A kind soul on another forum gave me some possible ways to mount the bed to the brackets, including a couple of links. It definitely pointed me in the right direction.
That led me to a trail of bread crumbs, which told me that I needed to measure the slot width - it's 8mm. Searching for an 8mm t-slot nut with an M5 thread led me to the fact that 8mm is the width of the slot on a 3030 extrusion. Searching for 3030 T-slot nuts led me to this,
https://www.ebay.com/itm/157340497226. Which allowed me to verify that these T-nuts had mostly the same dimensions as the ones that came with the clamps, the T-nuts in the link are M5 - the ones from the clamps are M6. (I've got enough issues with M5 screws, M6 would be worse.) One "Buy it now" transaction later and VIOLA, I have a plethora of T-nuts on the way. Although mounting the bed to the brackets, and the X-axis lead-nut to the bed, will take a healthy bite out of that plethora - I should have enough left over to allow me to clamp objects to the bed for years to come.
I also got my M5 grease zerks today, more stuff that can come off the ToDo list.