Very simple but great idea. The outer silicone lip is good enough for your lower lip. The inner is not the same size but rather just enough of a rim to cover the rim of a snow peak cup.
So this will not work well with other cups.
It sort of works on the gsi halulite, but not really.
Once i realised the inner lip covered just the rim, i expected this to drip/leak while sipping hot drinks, but surprisingly, it doesn’t. Works really well.
I bought them for all my snow peaks.