A lot can be done with Jenga style wooden blocks. They are available for little money. They can easily be Wood Glued together, to form larger structures. They can be cut/carved with an X-acto Knife, a box cutter, or by using a moto tool with a cut-off wheel. If you paint the wood a darker metallic color, they can easily appear to be metal constructs, at arm's length. They are available from Amazon, for cheap. Here is one video resource as to how these blocks might be used: YouTube link at 1:44.
Another option, is to use pink insulation foam. It is readily available, and relatively inexpensive. It is easy to cut, carve, and shape. Here is a crafting video on using pink foam to create dungeon stone walls with insulation foam blocks: YouTube link. Note that insulation foam can be carved to look like other materials, not just stone walls. It can be carved to appear as brick fascia, or brownstone buildings, or anything you can come up with. Big box, DIY stores now sell 4' x 4' x 1" or 2" pieces of pink insulation foam for crafting projects.
If you decide to go in deep, into insulation foam, a Hot Wire Cutter is highly recommended. The Proxxon Hot Wire Cutter is amazing, but it has some aftermarket accessories (three pages, be sure to view all three!) which are top notch, expanding its capabilities, incredibly. Search YouTube for videos which demonstrate what the Shifting Lands' Proxxon accessories are capable of…
These are just to stir your creativity with how these items might be employed to craft a variety of constructs and buildings. Cheers!
