Along with plastic welding and epoxies (good things to say about JB Weld) also using baking soda infused with cyanoacrylate glue (the very thin kind) can build

Along with plastic welding and epoxies (good things to say about JB Weld) also using baking soda infused with cyanoacrylate glue (the very thin kind) can build up material and becomes approximately rock hard. Can be filed. Just be careful around transparent pieces as the fumes can haze them up. There's also a commercial product called Q Bond that I believe includes a plastic powder as the "host" material but is used in the same way.