Here is another idea for mounting 2.5 inch drives without the expensive 3D prints. OCZ makes an adapter bracket (OCZACSSDBRKT2) available for about $5-$6 ea;
https://www.amazon.com/dp/B002I8MUU0
Used as intended, this places the SATA connector almost where it should be relative to a 3.5-inch drive, but not within tolerances for a backplane.
Instead, you can turn it over and mount the SSD to the back with double-sided tape. I used a 3.5 inch drive to get the depth right and line up the edge of the inverted bracket with the edge of the SSD;
I put one strip of tape on the left edge of the SSD and one on the right edge of the bracket, so they’d easily slide around to align, then pressed them together when aligned. There are probably some other ways you could attach the SSD to the bracket, aluminum is fairly easy to drill through.
When you slot it into the HL15 just be sure it is aligned with the SATA side of the bay and that it goes straight down, clearing the bottom bar and not going diagonal into the neighboring bay. Mine went straight down aligned perfectly with the connector. You might have to wiggle the adapter a bit to get it past the fingers, but after that it should go straight in like an HDD.
This should also work with the HL15 in tower orientation, and with 15mm drives.