Use a knife to take a piece of foam plastic used for packaging, and shape it into a roughly cylindrical shape, with both ends flat and slightly thicker than the diameter of the flute tube. Then use your hands to squeeze the foam plastic into a thickness that can be stuffed into the flute tube, and stuff it into the flute tube. The last step is to use a stick-like object to adjust the position of the flute plug inserted into the flute tube. No need for gluing. After gluing, it is difficult to adjust the position. It will be difficult to deal with any problems.

I wish you success!