‘Smart’ tray are not available… Ended by modifying the blank fillers…
I have removed the back of the filler (the part which gives some rigidity) with a sawblade.
Then I have drilled holes for HDD screws (approx. 4.5mm from the back, 0.7mm from the bottom) – 5mm diameter, then used a big one to hide screw head.
Here is the final result. To be complete something should go on the hdd bottom sides to compensate about 1mm height, but I have used this directly with no major problem. I would just recommend to push carefully when inserting the caddy in the disk bay.