I ruined a low whistle once, and learnt my lesson: thumb rests and neck slings are safer! Also, since they releave the fear of dropping the whistle, I believe they may improve one’s playing.
I don’t like the clip-on thumbrests made for swhistles/recorders by Aulos (and marketed by Susato in the US, it seems). There are short, plastic and “two-legged” i.e. protruding in a uncomfortable way under the thumb. Often, they become obtrusive for the right hand index hole.
Better see a woodwind repairman, get a clarinet thumbrest. They’re meant to be screwed in the wood, and the repairman can do it easily except with thin-walled (brass, copper) whistles. They may also be tied to the whistle.
Here’s an adequate and easily mastered knot–whipping, in English–also nice to attach a loop for a soprano sax sling and hook :

A carefully tied whipping, with a tastefully selected thread, may become quite ornamental, btw. Of course, they’re other knots even more ornamental, but those who know them didn’t wait for this… landlubbers’
thread, did they?
For a very tight fit, make it with a wet natural fiber thead, and it will shrink when dry. Then, varnish it for a permanent fit. Same may be used to fix a crack in a cane/bamboo instrument, too.
For an alternate hardware, go to an angling shop, and check out all these little rings they have to guide the line along a fishing rod; many are still made to be whipped to the rod.
Choose a small “single-legged rod ring” ; its loop will make a nice thumb rest and/or ring to attach a sax sling.
For a better comfort, slip over the loop this clarinet thumb “cushion”, made by BG and available from many woodwinds shops.
BG thumbrest cushion
