This article describes a way to rivet tenons to the little arch. The tenons here are made from roughly 5mm thick steel of round cross-section. The distance from a tenon to its neighbor should match the width of the field frame bar's pi-bracket as closely as possible.
The process Edit
Drill a hole that is slightly smaller than the diameter of tenon through the forks in the little arch. If you want the rivet head to sink inside the fork you can drill down a bit from the top with a significantly larger drill bit:
Cut a small piece of round steel to make a tenon. Grind or file most of the tenon to the same diameter as the hole in the fork. Leave a small length (~5mm) at one end untouched, so that tenon can't go through the hole entirely. This small length will also be braced against the pi-bracket in the field frame:
The tenon's end should jut a bit over the top surface of the fork. Hammer the tenon down to form the rivet head like shown below: