The kitebuilder discussion forum had a suggestion that sounded helpful for ferrules. If an internal ferrule is a little too small/loose one can paint it with fingernail polish to increase its diameter.
I haven't tried it but I did experiment with applying fingernail polish to some small diameter aluminum tubing and it seems to stick well. Additional thickness can be achieved by applying multiple thin coats.
The coats are very thin so this technique would only work for minor adjustments of fit.