Unless those are really high wattage LEDs, you’re probably going to want more of them than you have. Curing wants lumens and candlepower decreases with the square of distance. Your box is large which is nice, but the LEDs are much further away from the center as a result. You need more lumens to make up for distance.
Also, some resins (Flexible) need to be post-cured underwater (anaerobic environment). If you used a clear container you could put the LEDs on the outside and fill the inside with water when/if you needed to.
You’ve seen my post here? Check out how much closer each wrap of LEDs is on my design. The amount of light I’m getting on the print inside the jar is just getting the job done. Further away and spread out more, I would not have enough light.
Also. You appear to be using a RGB programmable LED string. This may not be doing what you expect. The “color” of the light is not what’s important, it’s the actual frequency of the light. That will exclusively be a function of the frequency of the Blue LEDs on the LED string. Unfortunately, the frequency you need (405nm) is almost purple in color. It’s unlikely to be the frequency of the LEDs on your RGB string. An RGB LED’s blue LED will typically be in the 470-480nm range. That’s too high to be effective at post-curing (if it has any effect at all, it may not).
The LEDs I recommend in my post are spec’d to output 385-405nm. They’re much closer to the 405nm target but are probably on average a little under. But 470nm will be a wavelength too far.