I've always put my motion in an aquapack but the last couple of times I've had to swim and try and catch the kit it comes down my arm and I 'm trying to swim with a handful of pouch.... I'm slow enough as it is..
I decided to put the armband on but I can't feed the strap through the holes. I'll get a narrower non elastic strap and use that .
I'm wondering how secure that method was? It's the model where you have to feed the strap right through one slot across the inners and out the other..

I've used the supplied strap and attached to my upper arm with no issues. After tightening the strap, I loop the tail of the strap around the base of the mini motion and then back under the tightened strap so it can't come loose.

and wear a watch on the same arm so if it slips down will be stopped at the wrist

Thanks. I've got to but some narrower elastic.