I decided to upgrade the case for my FrSky X20S and ordered one from AliExpress - https://www.aliexpress.com/item/4000677757547.html (size: 330x235x170mm). The original insert fits right in but leaves a 3cm gap on the side so i designed few different type of inserts in order to close that gap or make another compartment for gear.
Case Spacer Left Side + Case Spacer Right Side → simple gap closing inserts from both sides of the original insert
Case Insert cut sponge v1 → if you decide to cut the original insert on the left side, where the the neck strap located and create a big compartment on the left side of the case
Case Spacer single insert as compartment v1 → if you don't want to cut anything but a small additional compartment on the left side of the case
Case Spacer v3 -< Single insert from the left side of the original insert with no compartment
The author hasn't provided the model origin yet.