Look up one of the many micro indoor flyer plans. For example here’s a micro Pitts in 3mm foam http://www.chris3d.com/files/micropitts.pdf
If you are talking ultralights with monofilament wings etc then those are another sub set again, many use very simple controls, you can get micro actuators and very small motors and batteries for those. RC Groups forum has tons of stuff like that.
keep in mind that the smaller you go, the harder and more expensive it will get. the FT mighty mini's are about the prefect size for cost and ease of construction. They are small enough you could fly indoors but would need to be either a good pilot or have a really big indoor area.
The 3mm depron planes are not any more expensive to build than a mighty mini. There are numerous micro transmitters and tiny servos, all are fairly cheap. My mate used to make them out of pizza foam.
People were flying them in small gymnasiums long before Flite Test was a thing.