thermal glue or paste thermal resistance
Sounds like you're gluing a TO-220 package or something similar. 0.53 C/W (or K/W - same units) sounds quite reasonable. If you press real hard on the device while the glue is curing you might get down to 0.4-0.2 C/W, but that's probably as low as you can realistically go unless you have a really runny glue (0.1mm sil pads bottom out ar around 0.25-0.35 C/w with something like 5kg of force on a TO-220 so I'm using that as a guide)