Thank you TurdPooCharger.
Sadly, the powerbank is beyond a doubt too large to fit in one of the handles. I'm trying to decide where to put it at this point so any help is appreciated.
I'm wondering if you read through my post though, as I stated that I would be 3d printing the grip and you then directed me to Amazon to buy one.

 

I'll be printing the grip in an impact-resistant material known as PETG. I won't have any trouble making it smooth either, since I'll be printing with a layer height of 0.05 mm. In contrast, most libraries and on-demand services print at 0.2mm.
I'm thinking about mounting the powerbank in the center of the grip, with a kickstand that swings down to reveal the charging button. I'm wondering about balance though as the powerbank is quite heavy. Any suggestions there?

 

For my purposes it will be good enough. However, I'm also considering printing in ABS and then using vapor smoothing, which gives it a glossy look and the texture of injection molded plastic. I may do this once I've got everything finished in terms of prototyping and the design in the bag.