There is no 'best way'. Both Forward Components and Phil make an eccentric bottom bracket that will work with your frame. Not cheap. Paul, Surly, Yess & others make purpose-built chain tensioners. For that matter, chain tension with an IGH is not nearly so critical as with a fixed gear, and with a little fiddling with chainrings and cogs and perhaps the use of a half-link you shouldn't have too much trouble getting there.

Without knowing more about your commute and what handlebars (shifter) you want to use, it's hard (for me) to make a specific IGH recommendation.

Here and here is some inspiration.