This isn't specific to Linux (sorry) but I'm hoping someone will know of an adapter I can use. I can't find a cheap one on the Internet.

The issue is that the tray slides into a cage using side rails, so the 3.5" HD/bracket needs to be secured from the bottom. All the cheap brackets that I've been able to locate seem to want to mount the 2.5" SSD from the bottom and the secure the 3.5" bracket from the sides.

I've found some more expensive kits that want to be all things to all people, including providing a circuit board to move the SATA pins over to where they would be on a 3.5" drive. I don't want to pay for all that capability when I just need a simple bracket.

Frustratingly, I've found some adapters that look like they would have done the job except the designers decided to vent the bottom plate so I can't just drill holes in the correct locations.

Also, I need the vendor to be able to ship to Canada.

So I guess this does not fit what you want?

Since it has venting on the bottom. It is kind of hard to understand why you are having a hard time choosing.

Maybe?

I have a hard time to understand how your drive-bay is actually looking like. Can you provide photos of a disk with rails attached?

The first link shows the type that I have. Since there are no mounting holes for 3.5" drives on the bottom and since the 2.5" mounting holes are offset so they interfere with the 3.5" mounting holes, it doesn't work.

The second link shows something more like what I want except it's hard to tell if there is enough metal to secure the bolts from below into it. I've seen it in my web searches but no one seems to show a top or bottom view of it, so I can't tell if it would work.

The rails are riveted to the sides of a tray. The tray has mounting holes on the bottom with rubber grommets to allow a 3.5" HD to be secured from below. There is no way to secure a drive from the sides. It has to use the bottom mounting holes.

It's actually quite a nice system for holding 3.5" drives. It's just not so good for 2.5" drives since they can't be mounted directly with more than one screw because the 2.5" bottom-mounting holes are in different positions.

Not as inexpensive as the two devices in a previous post, but I have used the adapter listed below as a matter of convenience for mounting 2.5" drives in a 3.5" slot.

Western Digital IcePack

Thanks. That looks like a good unit but overkill for an SSD. One of the photos does show the holes for mounting from below as a 3.5" drive which is what I need.

I found a cheap plastic unit from Bytec that did the job. It takes two 2.5" drives, which again is overkill but probably doesn't affect the price. I don't like plastic because it insulates rather than dissipating the heat from the drive and I don't trust plastic to hold screws in place, but cheap metal units have been hard to find.