"Recruiters tell me that I'm a "job hopper" and that "it'll be difficult to place you."

Just explain that you have been contracting for the past couple of years and are now interested in something more permanent..or just look for contracting gigs.

You will most likely need to be more creative when looking for these types of jobs (contracting). Major job boards are filled with recruiters..which makes it difficult. Your best bet is to find companies that aren't using a recruiter.

"far out crazy idea for a hardware device involving Lidar that nobody else is doing"

This is probably not realistic. First find a way to actually make some money now.