A triangle bell that actually works is not as simple as it might seem. The triangle itself is best made from solid steel round bar at least 5/8” in diameter. Triangles made from smaller stock will not ring loudly enough for outdoor use. A bell made from 3/4 " solid round sounds better than one made from 5/8" but at some point increasing the mass brings diminishing return.
The triangle needs to have a large enough internal diameter so that there is room to swing the striker vigorously with a circular motion. 16” sides work pretty well.
The triangle must be left open and the two bends have a critical radius to produce the harmonic sound. If the bend is too sharp or choked it will diminish the ring - see illustration. Bending over a 5/8” radius works pretty well. Leave the triangle clean and simple and put any ornamentation on the bracket.
The triangle must hang with leather or other material that doesn’t conduct sound well. I like to make a special swing arm bracket so the bell can be moved out of the way when not being used.
Assuming the bell will be used by an adult, the height of the top of the bracket should be about 75". With a 3” leather hanger the triangle is a good height to strike.
Most of the examples I see are so small they must be intended for “in house” use or just decoration. The dinner bell was a frontier tradition and fitted well with our old time demonstrations at the National Tallgrass Prairie Preserve.