Google offers this page: Passive cooling for a 486DX4 100mhz? so I found my way here.

I am thinking on passive cooling for a 486DX4 100mhz for my vintage PC (which I want to make completely silent) and found some heatsinks which may be nice for this purpose:
https://www.micforg.co.jp/en/cat_pass.html

The two reasonable options seems to be the
* LPD 42x42x35, but that seems to be tested in a vertical position for natural convection.
* CS 40x40x50, but that is 5 cm high so it would take a 5.25' drive space. I think I will opt on this one as I will go for a Compact Flash drive anyway.

I probably go for Parker T412 adhesive tape (https://www.micforg.co.jp/en/cat_tapee.html) to attach the heat sink.