You still didn't show on what is the LED soldered onto.
What kind of PCB (printed circuit board) is the LED soldered on? Did you use Aluminium substrate PCB, or normal FR4? What thickness is the copper clad, and what copper surface did the pads have?
That type of LEDs can not work without radiator. Get some Al substrate PCB from a defective light bulb. If you don't have Al PCB at hand, you can find defective white light bulbs at any EE waste basket, or sacrifice a working light bulb you might have at home. Remove the plastic cap of the light bulb (there are videos online of how to remove the plastic cap without damaging the interior), and retrieve the Al PCB.
The Al plate to retrieve from the light bulb looks something like this (without the wires):
Snip away one of the white LEDs, preferably one from the middle of the plate. Do not try to desolder the white LED, that won't be easy (unless you use a preheater). Snip the white LED with a pair of pliers, then clean the pads one by one, and solder your UV LED instead + some wire on the other side end of the copper clad.
Use the biggest soldering-tip you have, and preferably use a preheater, the Al underneath is a big heat sink, normal soldering iron won't be able to melt the solder. Do not increase the temperature on the soldering iron, but instead preheat the Al plate at 10-150*C or so. That will help a lot when soldering your LED. I wouldn't use any more the UV LED that smoked the flux, because overheating shorten their lifetime a lot. Use a new one if you have.
Then, test how much is it heating inside your enclosure. If the plate gets so hot that you can't keep keep a finger pressed on the back of the Al plate, then it's probably too hot, more than the 60*C allowed by the datasheet. Make sure there is good air circulation, eventually poke some holes to your enclosure, for natural convection to lower the temperature. Without air circulation, any radiator will eventually heat too much, no matter how big the radiator.