For high paranoia while avoiding having to cast runes as a source of randomness, deploy airgaps - type your plaintext message on a disposable device, which you never connect to any network or removable storage. Ideally run it from read-only storage, so that your message only ever touches volatile memory. Run the encryption and copy out the encrypted version (ideally by hand, or maybe by print+OCR if that's impractical)
Afterwards, ensure that any trace of the message is gone by repeatedly overwriting the contents of memory. For maximum paranoia you ensure that the memory isn't readably by running it throuhg a woodchipper, collecting the fragments, and sealing it all in epoxy which you then encase in concrete and drop into either a deep unmarked hole in the middle of nowhere, the depths of the ocean, or the mouth an active volcano... or launch into the Sun if you've got the budget.
Throughout, be vigilant for side channels - maybe the image you installed on your airgapped computer was compromised, and it's finding some creative way to communicate with the mothership. Maybe it's modulating CPU usage to make the temperature of your room fluctuate (detectable via IR), or maybe the noise your fingers make on the keyboard can be picked up as subtle vibrations that a sensitive laser pointed at the window can detect.
And of course, to be safe against goons with a $5 wrench, you also need to have forgotten the message and the key yourself. I recommend either wiping it from memory with a pint or two of lab ethanol, or extending the concept of a one-time pad to the human brain, by lobotomising yourself after sending.