File this under Lockdown boredom and trying to learn new things. I'm still battling the toolset to try and figure out a way to make this fire automatically (or add it to something clickable), but for now you can add the letter by downloading the mod and then typing runscript zz_giveletter into the console in game and hit enter. This will create a codex entry under notes (numbered 262) that you can read. There's nothing origin or gender specific in the letter so it works for all wardens, there's also some mushy bits towards the end (not pictured) but nothing saucy.

Please note this will NOT work alongside either version of Awakening letter from King Alistair since it references the same codex entry - but I'm working on that. When you boot up Awakening make sure you only have one letter mod in your override folder or it won't work.

IMPORTANT: There have been reports of this causing issues with other codex entries, nothing game breaking, but I'm investigating.

Similar mods
If you're looking for a letter from Zevran, Sten, or Leliana then Firinneach's Letters from Origins companions in Awakenings has you covered - although I believe the official BioWare versions do now deliver correctly from a romanced Leliana and Zevran without any mods used. I just need to figure out how BioWare managed it.

Feedback
Comments or suggestions for the actual contents of the letter are welcome (as are tips for the toolset!) and I'm happy to explain how I bodged this version together if anyone wants to make one of their own from any of the companions or NPCs. Additionally, there's a separate download available with just the text from the letter so you can have a look/make suggestions.

Using the console
If you don't know how to use the console then instructions are here. I'd also recommend using the mod Make CONSOLE commands visible  by monkeysnest so you can see what you're typing.

NOTE: I'm aware of the issues with ID mix-ups and I am still fiddling, but time's limited, so not sure how long it will take.