So I tried my hand at drawing a more elaborate Celtic-like knot. From some examples online, I picked out one that looped around a diagonal square. Annoyingly, it turned out to be a link, not a knot. Fairly easily fixed, it seems, just by connecting the central loops into a square.
Then I decorated the outside with some trefoils, and the remaining space with some church-window-like fused circles. Came out ok.
Although actually the geometry on the sketch looks slightly nicer…
The final product is on Etsy.