I posted the above questions quite a long time ago. I got a lot of good advices. I "finally" finish making the box to enclose the noisy shop vac (from Rigid). The result is very good in term of sound reduction - around 9 to 10db reduction. I have also replaced the old sound reduction device (from Rigid that plugged into the air outlet) with a version that uses spongy thing to reduce noise (also from Rigid); this buys me an additonal 3db sound reduction.
The shop vac doesn't seem to run hotter than normal.
Having said this, I don't mean to recommend people to build a box to reduce the noise from their shop vac. There are several problems of using the box: (1) This takes time, money and effort to build the box when we could have built other things (2) The box takes a lot of valuable shop floor space (3) The cheap $13 spongy thing from Rigid may be good enough for some people who don't need to cut down the noise that much (4) At the end, it can only keep the noise in the range of 67db (not as quiet as what I want) because the shop vac from Rigid (an old model) is very noisy to begin with.
If I would start this over with, I might get an expensive but very quiet shop vac instead of building the box.
Jay Chan