Metal internal 2.5-inch SSD/HDD mounting kit (for up to 2x 2.5-inch drives per 3.5-inch bay) by NEON

While these are sturdy and perfectly fine, they seem to use non-standard screws to put them in the case. Not a major issue, more an annoyance (though that may also depend on the type of bracket you have in your case...)

I have 2 cases with caddies that require attaching a drive by 4 screws into the bottom. For the life of me I could not get these to work. You need to mount through the sides, also space is a bit tight if your SATA cables have a big clip on them or an L end you'll run into trouble, other than that, if you mount your drives from the sides and have simple straight cables you should be happy enough.