For firm slabbed ganache & gianduja, I use the stainless frames from design & realization, mostly 15mm but I have a couple of 10 mm as well. Pate de fruits can ooze under the edges a bit but they work. I don’t think caramels work in those frames, it stays too hot and runny for too long. Instead, I use 9” square cake pans lined with parchment for caramels. False bottoms a plus if you go that route.
How many pieces do you want to make at once, and do you already have a guitar?