A few years back I was looking for an alternative to the Burger King toy C3G was using for their green construct. I picked up this toy set at Toys'R'Us, took one of the figures, and spray painted him green.
You can see the figure in play in this photo:
The figure came with this big gun that could be mounted onto his shoulder and would rotate around. Since there were two of those figures I had two big guns. I simply rotated the bottom facing down and glued it to a standard base. When I place them on the building they stand straight up and look like really cool pieces of Baxter Building technology. When they are destroyed I bend them over so they become ITFs and can be picked up and throw or used to smash with.
Here you can see the gun mounted on his shoulder:
And here you can sort of see the two "towers" in play:
I sort of left what to use to the person building the custom. A simple set of narrow, black straws (like from a Panera Bread) glued together would work just as well. You could even make a working one and a destroyed one for that matter!
Hope that helps.