This right-angle square guide clamps right into the t-tracks under the rails. I've also added an adjustable stop block on the top to prevent tipping. 

For the clamps, I'm using the Kreg Tool KMA7520 Track Clamps:

https://amzn.to/49SCSM9

To attach the stop block, I'm using an m3 screw with a nut:

https://amzn.to/4a1IbZy

and washer:

https://amzn.to/3N7xzyF