What is the simplest "evil maid" executable out there?

Prerequisite: I have access to someone windows AND osx machine, both unlocked, but only for very short period of times (5 minutes windows)

Goal: at a minimum, keylogging to a vps somewhere. At best, intelligently monitored usage breaking down activities by category: emails, web history, document creation, I/O on drives.

Challenges: cannot be picked up as malware.

Price: willing to pay, but would prefer FLOSS, evidently, so the source code can warrant no data flows outside of the chosen VPS.

Question: what would be the most optimal/simple way to achieve this.

Thank you.

For information, I have access to a rubber ducky, and a vast array of hack 5 devices. They all present the same disadvantages:
a) those that act as permanent devices are easily spoted on a laptop. So goodbye packet squirrel, for example.
b) those that don't, like the rubberducky, are designed to inject payloads, usually for things like windows credential theft at high speed. Sadly, I'm not interested in these - I'm interested solely in installing a RAT for future remote access via a proxied server.

It's been 6 months I've been trying to achieve this and NO ONE has ever even pointed me towards a resource to learn. It's incredibly frustrating. I have 40years of coding experience. The problem is that I have no experience with RATs, as the vast majority of those that are easily available are sending the data in all sort of nasty places (careful when it's too good to be true, usually is).