This key has been tested on the following models and years:
2007 Civic
2009 Civic
2nd gen Honda Fit
2014 Odyssey

It will likely work on all 8th generation Civics which run from 2006-2011. If you have success for a year not listed, please let me know and I will add it to the tested list.

I found a housing here on Thingiverse, but it kept breaking. I decided to make a new one from scratch in the hopes that I could make something more resistant to daily and sometimes rough use. Its thicker and longer than the one noted below:

https://www.thingiverse.com/thing:954099

This is not a remix of the above. It's a from scratch make. Having said that, I do want to thank the original author for the inspiration.

This version has thicker walls and hopefully more "sandwiching" of pieces around the thin steel T part of the key. (Dumb design, btw, Honda.)

I have included some images for how I orient the parts in the slicer to make it more resilient. The top face is printed flat, but the back part is printed standing up to get the layers going in a different direction than the key turn torque force. Printing the back flat will increase the possibility of it splitting if you don't have good layer-to-layer adhesion.

I also found using ABS to be more brittle so I have used PETG for this print. Haven't tried PLA. I have also included a screen shot of Cura's settings for this print.

The bolts and nuts used to hold it together are three M2.5 x 20mm. (It looks like some fobs actually use some 2M screws that seem to work too.) You will have to cut the ends off the bolts to make them flush with the back. To help stiffen the unit, I have put nut traps in the bottom so that you don't have to use screws which may let go under torqueing forces over time.

I have hot glued the key into the housing for some added support.

Hope you have found this useful!

UPDATE DEC 15, 2021:
Added another STL by user request that doesn't have the red button. The new STL is called fob-front-no-red-button.stl. Apparently, it's for a Honda BRV. I haven't tested it myself, but hopefully the user reports back with success or fail.

I also added a three button version as requested. I don't know if it exists, but for completeness, I added a three button with no red button derivative too. They are called fob-front-three-buttons.stl and fob-front-three-buttons-no-red-button.stl respectively.