The airflow is going to be the main thing, as mentioned, as 3d printed boxes have infill beyond the perimeter layers, so they are quite good insulators. I would not use PLA for this application, yes it shouldn't outright melt at those temperatures, but it does soften slightly, and if anything presses on the top above the heatsink, for example, it will slowly bend and eventually make contact.
You could try using PETG, it prints on most printers that will run PLA (not much higher temp) but is a bit higher temperature and a bit more rugged. That said, my printer's extruder housing was PETG and after a few months of use in a 50C heated enclosure, the parts under mechanical pressure had bent enough to the point of being unusable.
I would design so that you have a clear airflow path out (looks reasonable for that) and so that you stay clear of the edges of the heatsink - something like a horizontal mount may be much safer for higher temperatures just because of the clearance to the nearest side. From there, it's just up to testing. You can get something that will read a few thermocouples or thermistors and go to town with running it in various configurations. I'd estimate the box as it is will be good for less than 10W even when optimized unless you can guarantee that the airflow slits are unobstructed or you have some forced air. If you, for example, made the lid out of metal and thermally coupled it to your heatsink, you'd easily get a couple Watts more dissipation, but you'd have to worry about the temperature of the interface of the lid to the part.
I think you could simulate it with free software (don't have specifics, but I know it exists), but I think the simplest method is going to be trial and measurement, and then building in a margin so you don't run into problems.
If you have a capable printer, you can look at higher temperature materials too. Something like ABS, nylon, or polycarbonate holds up to the heat better.