Well after about a week of not doing much, waiting for a brake line order to come in, and waiting out a heatwave, I finally started sanding down the lip in the seat tube. I was anticipating doing this for hours so I decided to wait until after 100+ degree weather here to pass.
Much to my surprise, the lip took all of 10 minutes to sand down enough to fit the Di2 battery through. I believe SPCycle really did listen to prior feedback about internal batteries and addressed it for the most part.
My solution was taping a file I picked up in a set from Harbor Freight to a paint stirrer I got for free from Home Depot. Worked out nicely.
I just worked out the internal cabling for the Di2 wires and the rear brake. Next up is the cabling through the handlebar and then finally the front brake routing. Then it's time to grease everything up and torque everything down. I'm excited to finally finish this build!!