TBH when you were originally talking about cladding I was thinking more in terms of straight vertical pieces which is the most common type I've done.
If the problem is that the square us slipping you could always glue a strip of sandpaper onto the edge of the lip on the square and if needs be run the timber up against a couple of stops. If your angles aren't 90 and 45 degrees there are alternative (adjustable) squares available:
View attachment 290494
Or maybe even think in terms of a simple home made mitre jig which uses either sandpaper or screws filed to chisel points (oriented to the grain) to grip:
View attachment 290497
Or is your issue something else, such as spelching or break-out?