Bitbelt 12 pack (one of every...
https://www.amazon.com/dp/B010OX42WO?ref=ppx_pop_mob_ap_share
Buy these before you buy the MagicBand+.
These work. They are a one piece (not a clip) silicone sleeve that stays put. We used them successfully all day at
Disneyland. They are technically a TINY bit too small for MB+, which is actually a benefit as it keeps the bands secured really well. Once you have it on, you don't feel it at all.
But yes, without these the bands will pop off all day. The mechanism to keep them closed is very weak and the band material isn't as conforming as the old style magic bands. It's more of a stiff plastic that wants to return to its straight shape.
Universal gives these away for free for the Super Mario world bands. Because those fall off all day too (they are snap bracelet style but don't fasten closed). Disney should include them with bands also, since their clasp design is flawed.