Check out this custom bladder tank I built for a friend’s Sebart 1.9m Avanti. Tailored to fit the fuselage space, it features a bottom-wide, top-narrow design that hugs every inch of the internal area. Internally split into two compartments—one for fuel and a smaller one for smoke oil—we’ve maximized the use of the limited space.
It’s manufactured in a cleanroom using specialized welding equipment, ensuring the weld strength exceeds that of the material itself. A dedicated plastic fuel nozzle is integrated with the tank body, reducing weight while eliminating the risk of oil leakage from metal nozzle seals.

Looks like single layer bags?

Yeah—there's no difference in manufacturing difficulty between multi-layer and single-layer versions. But the single-layer one is lighter, and its strength is more than sufficient. I’ve always used the single-layer design myself.

My point is that single layer isn't approved for use in the US, at least to comply with AMA rules which means no flying at clubs, events and no insurance. Not sure where you're located so that may or may not apply to you.

You're absolutely right. I'm based in China – there's no requirement here that rc fuel bladders must be multi-layered. However, we do often manufacture multi-layer fuel bladders, but they're usually not for rc aircraft. Instead, they're mainly used for high-altitude UAVs.