This article shows how to rivet fairly thick pi-brackets (here 5mm) to the field frame bars.
The process Edit
First drill (or punch) holes for the pi-brackets to correct places:
Cut the ends of the pi-brackets reasonably short, so that they mushroom instead of bending. Then just hammer them to form the rivet's head:
The shoulder of the pi-bracket should be in level with the field frame bar for maximum robustness:
Here are a few pictures of field frame bars with riveted pi-brackets: