Recently got into Adeptus Titanicus.
I bought a few 3D printed ruined buildings to supplement the stuff that comes with the AT Horus Heresy starter set.
However I didn't want to spend any more money on expensive GW terrain sets or resin stuff.
I didn't think at first that I had any bits or leftovers that would work for 6mm scale. But then I remembered that I had kept all of my bits from when I built my Primaris space marine army for 40k.
Specifically, I had numerous 28mm bits from the Primaris Repulsor Tank and the Stormhawk Interceptor models.
To my great surprise and pleasure I realized that the bits from these vehicle models absolutely screamed 6mm! So while staring at all the various parts, I tuned my brain to think in 6mm, got some MDF and round plastic pieces and began playing around.
After a few hours, I had a pretty cool looking set of 6mm Sci-fi terrain.
Using 28mm engine exhaust parts, cockpit parts, bits of bolter guns for antennas, armor plating, space marine shoulder pads, etc. I was able to make a headquarters-looking kind of building, some storage tank facilities, hangar facilities, communication installations, a missile battery installation, and a couple bases of ruins.
All in all a pretty effective use of bits from a different scale, I think. And you can't beat the price! :)