The extrusion size is not the standard “Euro” size for 20mm extrusions. I’ve found that 80/20 Inc has compatible extrusions and hardware. Here’s the T-nut I’m using currently – it’s a bit expensive, but it a “roll-in” style that does not need to be inserted from the end. It’s also available in 4mm and 5mm sizes, but I’ve not yet tried those sizes (3mm has been completely able to support my LED lighting for the printer bed.)