1/4"-20 Threaded Steel Roll in T-Nut with Ball Spring, T-Slot Nuts Accessory Hardware Nuts for 10 Series Aluminum Extrusion Profiles Rail (20)...

These are worth their weight in gold if you work with aluminum extrusion frequently. I consider them a bit of a luxury, as they are considerably more expensive than the economy t nuts. Where they really shine is when you forget to put some nuts in while assembling something, or when you need to add something after the fact. Because they just rock into the groove, you can slap them in anywhere you want. The ball holds it from sliding around, so you can position the nuts accordingly, then attach a part without having to use a zip tie, or bamboo skewer to slide the nuts into place. These are considerably more economical than the big name brand fasteners, and every bit as good! I approve whole heartedly!

Not as big as the ones I have bought before they seat a little crooked. More like they are made for 20mm?

I got these for 1010 extruded aluminium and they fit in the rails without issue and hold tight. Haven't had any problems with loose fitment or them working their way loose over time as of yet.

These fit my 10-series extrusions... mostly. They're a little shallow and the spring-loaded ball doesn't really do a good job of pushing it towards the opening. I also wish they were a little wider to get more contact area for a stronger joint. They work and there's nothing really wrong with them from a functional perspective but there are better options out there.

This is a nice pack of T-Slot nuts for 1010 style rails (1-inch square). The balls are a very nice feature that helps the nuts glide internally. They are properly sized and nicely finished, no sharp edges. I do recommend this product for your consideration.