I wish FFG would make ground vehicles for the X-wing game, but so far they have not. This set of files will allow you to add AT-ATs to your game. I will leave the rules up to you, but for casual/scenario games they are quite fun.
First is an X-Wing style base. It is the same dimensions as the FFG 'small' fighter base but it is double length to accommodate the longer model. There is a front to the base, indicated by the 90 degree lines which denotes the firing arc. I just lay the unit number on the base between the front and back legs.
Second you'll need an AT-AT model. I used this excellent one by Kolo33:
I did not create the AT-AT model but I made five of them and they turned out well.
I used the fully assembled model, not the individual pieces, for my prints.
Print the Kolo33 model at 40.5% scale. Wookiepedia tells me an X-wing is 12.5 meters long and an AT-AT is 22.5 meters tall. If the toy X-wing is 1.8" long then the AT-AT would need to be 3.25" tall. 40.5% will get you there.
Third, you might need to replace the cheek guns on the sides of the head. While I was cleaning off the supports most of them broke them off. This is not the model's fault, it was never intended to be printed this small. So, I included files for replacement cheek guns should you need them. The separate cheek guns Kolo33 included were too delicate to use; again, not the model's fault.
Also, I cut the heads off of each model. Two reasons: I wanted to re-position them a little bit so every model was different, and, I wanted a very clean print for the head with no support residue. If you print the head separately you can do it without supports. Kolo33 was nice enough to provide a full model and also the model as separate pieces, making this possible.
Finally, I included a 3.25" tall cylinder should you decide to use a model other than Kolo33's. First, place the cylinder on the virtual build plate of your printing software. Then drop the AT-AT model on the build plate. Keep adjusting the scale of the AT-AT until the top of the cylinder is at the top of the AT-AT's back. Finally, delete the cylinder and print. This way you know your AT-AT will be 3.5" tall; the correct scale for games of X-wing.
This file is a free download. Since it is listed as 'creative commons - attribution' there are very few restrictions on how you use these files. You are welcome (and encouraged) to print them out for yourself for personal use. You are also allowed to buy and sell physical copies of the models. However, if you use my files and my pictures you should provide a link back to where you got them.
Please note that these permissions only apply the files that I have posted. Kolo33 has his files (last I checked) listed as "non-commercial" so please respect his request.