I’m the guy selling the battery in the seat post on eBay. What seems easy to one is not so easy to all. That’s why I started selling the wiring kit(s) on eBay. For some people just being able to buy a package that is plug-and-play is the easiest way to go. I started looking on the same forums, for the same information I see here, and after a year, and doing > 20 frames, both Di2 specific, and non Di2, I would say now it’s easy, but not in the beginning. I first looked at the Calfee retrofit, but after some research I decided to do it myself. Calfee does a great job, and if you want a bike that has had holes > ½ inch drilled, and cutouts the size of a fist drilled in the downtube, and a harness installed in the frame that cant be removed... and if you have an extra $700 plus shipping, than send that bike frame to them. They will get it back to you in 4-6 weeks. Yes they do a wonderful job of patching the big holes they make, and make it almost unnoticeable, if you don’t look too close. If you have a Di2 specific frame it’s realy easy to get rid of the ugly battery, and put it in the seat post, all you need is an internal harness and a battery that will fit in the post. I also just noticed that Glorycycles is selling a Calfee Enve Seatpost 27.2 with Di2 Battery, only $399!!. If anyone is interested in doing a frame that is a non Di2 frame or a Di2 frame, message me and I would be glad to help you drill that puppy and put the battery in the seat post. If you live in So Cal you could bring it to me and have it done in a < a week.
Just ordered this, and will report back when I get them installed. Ordered two medium size from "di2diy" for my Cervelo R3 and my friends Look 595. Both bikes have been internalized (http://weightweenies.starbike.com/forum/viewtopic.php?f=3&t=86592#p752149
), and the last step was moving the batteries inside.