No overheating, but you need to insure you have a large fan, and enough air space between the box layers to allow for good air flow. It takes up space to do it that way and I suspect people that have tried did not get a large enough air flow area or their fans were too small. The air gaps where the air flow runs can be larger than those on the other sides of the boxes. Sound does not like to go around corners, so its like making a maze, to let air in, but not sound.
To monitor the engine air temp you can use a marine type temp gauge for an engine compartment. A lot of what we’ve done in this area is taken from marine designs and know how. If you don’t have the luxury of the necessary space, then try and use isolation mounts for the gen, and perhaps two walls. Don’t dismiss isolation of the unit, as a lot of the noise is transmitted via vibration. So rubber or spring type mounts help a lot. And isolate the inner box also. In the motion picture industry its called blimping, you might find some info by googling with that word.