Detail of the canister attched the to rail. The lip of the canister against the rail is all I needed to allow me to use a length of wire cable to hold the canisters up. Getting the length of the wire cable was the only tricky part - tight enough to hold the canister in place but loose enough to allow the loops to slide along the rail.
Luckily the Gruntal bars and brackets are separate, they mount together when you put them in place. Having the bar free made figuring out the wire cables easier.
I used wire cable and ferrules from a hardware store. I don't remember the size, but it was in the aisle with the big spools of cable you have cut to size, and it was the smallest size they have. I found it challenging to cut the wire cable using regular wire cutters, so if you decide to do something similar you might want to look into cable and ferrules at a jewelry supply shop.
First I wrapped a loop of wire cable around the bar and through both sides of a loose ferrule. I marked where the ferrule sat over the cable using a permanent marker. I made the loops pretty tight (the cable wouldn't curve much more than that), I didn't want the ferrule to sit against the side of the canister.
I took the wire outside and used a flat head screwdriver and a small mallet over a stone in my front stairs to clamp down the first ferrule over the marks. Place the flat head screwdriver over the crease in the ferrule and pound, this will tighten both sides of the ferrule.
Next I put a canister in place with the lip over the edge of the Grundtal bar, wrapped the wire around the canister, and repeated the looping over the bar on the other side using a loose ferrule. This was a little awkward. I needed to be sure the cable was tight enough to hold the canister in place, but had enough slack to allow me to slide the attched canister back and forth on the bar.
I marked where the ferrule would lie on the wire cable, and where I wanted to clip the end of the cable and took everything apart. I clipped the cable, and took the wire outside to clamp down the ferrule to create the second loop. I made a little hopeful gesture, and tested it. The lip of the canister holds it above the rail, the bottom of the canister sits against the wall. It works really well for my purposes.
I did consider using utensil canisters that have holes in them - see Ikea's Ordning for an example. This would have made it really easy to use some bendable wire to attach the utensil holders to the bars. But I just didn't like the look of utensil holders with holes in them, and they also create lots of opportunities for poke-through.