I have sourced a supplier who can make perfect 5200 MAH batteries that fit inside the battery bay perfectly and even has spare space. I am currently running my Pi 3 off it with no issues using the Banggood 1A charger. No power issues atm!
I was hoping to find interest in Australia. Who would like to purchase one of these batteries. They have everything, the JST connector, proper tape and balance circuit pcb. I have confirmed they are not new old stock and are very recent batteries. Seller is very good at providing high quality batteries. I will confirm the Mah soon and the time it takes to run out of battery completely till dry.
Please excuse my bodge Pi 3 - It is a prototype.
Please let me know in a post below. If we do go with this and we get about 10 participants we can reduce costs. Shipping within australia should be cheap. I assume about $5 and it is legal to send within Australia (already checked and confirmed!)