Exactly like Oggy said above.
I prototyped a dual 18650 design but didn't like the 'brick' feel to it.
Was doing my best to keep it about the size of a pack of analogs (something most ex-smokers are familiar with), and this design is just a tad thicker but almost exact in height, and width.
I was a bit concerned about battery life but after just vaping the snot out of one of the version 11 prototypes at 50+ watts (torture test really chain vaping it every time I pick it up. I went through ~50 ml of juice in about 4 days) I found the battery life to actually hold up quite well.
I do need a battery change about halfway through the day or more recently I connect it to a 6600 mAh power pack between vaping sessions to keep the battery fully charged.
With the 1 amp charger it charges up quite fast, and with the sliding back panel battery changes are a less than a minute tool less operation.
In the end I am willing to carry an extra battery or power pack to retain the small form factor of the PV.
On another note I have been kicking around the design ideas for a 26650 VW bottom feeder
One thing at a time though