olnico

This depends on the shape of the structure/ skin around it.
On circular shapes with a radius close to the bladder balooned state ( full ), then velcro strips around the bottom work well.
If the surface around is flat, then we use carbon fiber rods that slide in the side tabs. Because the bladder inflates considerably like a baloon when full, one cannot use velcros that would unfasten on flat surfaces.

There is a considerable amount of considerations going into bladders design. I have been making and using very large ones consistently for the past 30 years on RC jets and UAVs on our European operations.
For example, wing bladders are very specific in that they should not blow the skins out when full! Also, fill and pickup point are very specific. FInally, a fueling system with pressure sensor and bubble purging cycle has to be used.

There is an aerospace company called ATV that makes bladders in California for race cars and space vehicles. However these are completely out of the hobby range in terms of weight and price.