I always cut mine in half - not had a problem yet.
As for trimming them when fitted, as they are half the original length, not as issue. There's usually about 5 to 10mm sticking out, this gets flatted on the road and eventually falls off - the good thing about the worms at the moment is they fray and splay like a shoe lace without a ferrule and this I think aids them self-trimming to the tyre surface. I'm not sure you would get this effect with string or old inner tube - but worth a try.
I've never had one work it's way loose - there's enough both inside and outside the tyre to secure it.
Personally, worms are so cheap and seldom used that I think a homebrew version is not worth the bother, but happy to be convinced otherwise