In the late 70's I got a set of flatless tires, and I used them for several years. They consisted of the heavy triangular shaped tire with a flat rolling surface at the top of the flat topped pyramid of rubber.The rolling surface was about 3/8-1/2" I think. On the inside of the rim you placed a section of very heavy polyethelene tubing-maybe 1/2" outside diameter- 3/8" inside diameter. It was a real bear to get the tire over the rim. You had to trim the tubing to size, and then tape the two loose ends.
The ride was harsh, but the rolling resistance was pretty low;I don't remember it being more than a normal high pressure 28mm clincher. The main downside was the rough ride. A second slight downside was the rims would get knocked out of true much more quickly.
All and all they were pretty good. I'm surprised that nothing better has been developed in 30 years(I shouldn't be ,since car tire folks have been trying for almost 90 years since they switched from solid tires).I took them off once I didn't need my tires to be absolutely flat free(I was commuting). The ride was very harsh.
Luck,
Charlie