For me, this seems to be an "old" topic with various discussions (e.g. https://portableapps.com/node/35692), but no recent updates. I also googled around about, and it there seem to be various trials available, which are all outdated.
Actually I have to use a very restricted PC (no local installation without IT dep. confirmation), so I want to use cygwin via an external device (and also only to download/install it only once for my three private laptops )
As far as I understood various posts, Cygwin is free software which can be ported to portableApps?
I started to work on creating a portable app for cygwin using an actual setup.exe and concentrated on a base installation (bash/mintty). I ran into the problem of accessing HKLM\SOFTWARE\Cygwin on an 64bit machine (see also https://portableapps.com/node/54056), but up to now I can handle everything in Custom.nsh, so I don't use [Registry] in the CygwinPortableSetup.ini.
I'm actually testing storing and restoring HKLM\SOFTWARE\Cygwin\Installations and have some ideas to update appInfo.ini after installation to add a shell entry.
Looking around in the Cygwin Forum for the HKLM\SOFTWARE\Cygwin\Installations, I found the following entry:
According to this, there will always be a HKLM/Installations/ key generated (each time a cygwin session is started, and it's not deleted when closing all tasks) - so according to my understanding of the guidelines it can't be made portable.