Earlier this year, I decided to make my own version of Delta Sim's Slew Mod as a way to keep my engineering skills sharp. I've been using it for a while now and its been excellent, so I'm finally publishing my work.
https://github.com/ServError/Thrustmaster_Warthog_Throttle_Alps_Slew_Mod
What this isn't:
This isn't likely to be a way for you to build a slew mod for cheap. If you factor in time, equipment, and skill requirements, Delta's offering is well worth it and you get the benefit of customer service and supporting a member of the community.
What this is:
If you have ideas on how you'd like to do the slew mod a little differently (different stick, different sensor, etc), this project should act as a good source of files to help you experiment.
Enjoy