I've burned a Mac by getting pissed off with the harddrive noise and covering the thing with a cardboard box stuffed with glasswool. The processor burned, expensive joke. Apparently the fans are not there for nothing
Apparently some PC's have large heatsinks and are capable of withstanding pretty high temperatures, others are not.
One solution is to build a box, solid sides, top and bottom. Front top 2/3 solid, rear bottom 2/3 solid.
The open spaces, front and rear, make some slats, 2" wide ply, spaced 1/2" apart. At 2" dept, do the same, overlapping the first ones. Fill the 2" gap in between with a thin layer of glasswool.
This will draw some air in from the front, and release hot air at the rear, using the fan of the computer. It works for sound control.
You can line th inside of your box with glasswool. if you don't want to use glasswool get a sheet of 1/2" soundboard for about 7 bucks and glue 2 layers of that on the inside of your box. Soundboard does not retain heat as much as glaswool does.
If you don't want to take any risks with your computer, just buy a "silent" fan from any computer component supplier, and build that inside the top rear of the box instead of the slats. Mount it from the inside, after you have installed the soundboard (so in fact you have a small tunnel, with the fan behind it). You can hook up the power from the fan from the same wires that power your computer's internal fan.
Now you also know why I have all my computer gear, hard drives etc in another room...........