OP3/3T filler/insert for weatherproof case

Last year, I bought myself a mountain bike and a couple nice el cheapo accessories to go along with it. One such accessory is a weatherproof zippered case by Gear Beast:

https://www.amazon.ca/Waterproof-Holder-iPhone-Galaxy-Phones/dp/B00ZTJ8LWG/

Case is nice, but even with the (included) foam inserts, my poor OnePlus3 would be WAY too loose inside the case. And by simple eyeballing, I figured the case was thick enough it could accomodate TWO OnePlus3's.

So, personal case scenario: I hav