I think a dirty bomb would be more than "scary" enough, given general anti-nuclear phobia, and is still objectively far easier than even a simple gun-type weapo

I think a dirty bomb would be more than "scary" enough, given general anti-nuclear phobia, and is still objectively far easier than even a simple gun-type weapon. OTOH, I'm pretty sure I could build a gun-type weapon with pretty modest resources. More than Kickstarter, less than $50mm. The hard part would just be doing it in secret, especially since there are dual-use controls and people trying to actively find people making covert weapons. However, almost all nonproliferation controls are still based around restricting access to fissile material; if you solved that, I'd be comfortable doing this in a third-tier US city (away from the border), maybe in an old mine.