Ideally, you’d want leather covered clarinet pads (thin) sized to fit the key cups. Then you fix them to the cups with an ample amount of shellac, which you then heat with the keys in place so that the pads can float into their idea alignment. This means that you would need to push on the key touches with just the right amount of pressure that you would use while playing, while the shellac is still hot and liquid.
It takes some finessing to get this right. Also, depending on how the touches work, on some flutes the C and C# touches interlock and you need just the right thickness of cork between them in order to sound the C (which requires neither key pad to be leaking when they are both closed with even pressure).