So, here's a screenshot of my Tinkercad design.
I took the basic stop and added tongues to help guide the stop in the aluminium track. Since this version I've made the gp smaller to see if that helps keep the T-nut from turning when you try to locate it in the track.
The flag stop has a simpler block with a hole and recess for a hex nut in one end. The idea being that you can glue or friction fit an M5 nut or bolt head in it.
By chance the hole behind the hex recess in the end is tight enough to screw an M5 bolt straight in, so no nut needed. But it might work loose over time so a refinement is probably needed.
At the moment I'm printing a new version with a recess about 6mm in for the end to take a nut dropped in form the underneath.
In use the stops and flags seem quite sturdy. There's very little deflection in the flags