I have used the H2O waterproof holder for my GT31,s for years there the only comfortable holder that doesn't dig into your Bicep and have lasted ages but cannot seem to find them anymore. I think I purchased on Amazon initially but soo long ago.
Any ideas where you can purchase or worst case an alternative?

H2Oaudio was bought out, and the phone arm bag is renamed Amphibx. I cant find the original design now but they have a similar but larger design :

h2oaudio.com/products/amphibx-waterproof-smartphone-case

Edit:

I found a website listing the original design:
www.g2a.com/en-au/x-1-powered-by-h2o-audio-amphibx-fit-waterproof-armband-for-smartphones-black-i10000188678001

But you would have to use a USA freight forwarding service as they say they dont ship to Australia.

Here is another in the UK: They may ship to Australia but I cant find the info

order.komaks.com/x-1-h2o-audio-xb1-bk-x.html

Ebay US seller for the original smaller one that suits up to 4" Phones.

www.ebay.com.au/itm/254934961799?hash=item3b5b4ece87:g:CCEAAOSw-vpfDxwV

Try Aquapac.

aquapac.net

decrepit said..
I've always used paqua midis but they seem hard to get now. They were for smaller old phones, but phones have grown.

Fully agree, the Paquas are great! I use it for the gyro1, very compact such it fits also the car key in there. Since it is keyless entry I can easily lock/unlock the car without removing it.

kayakshopaustralia.com.au/aquapac-217-waterproof-gps-armband-case-medium/

My Paqua packed up this week after many good years - tried to get a replacement but they're not available any more it seems. So I got an Aquapac instead...seems good so far...actually easier and a bit more comfortable than the Pacqua...oh and I use a GT31...

Has that gone up in price? I seem to remember a figure beginning with 4 not 6

Surfsailaustralia.com.au also sell them but are currently out of stock.

I tend to make my own cases out of Perspex