Making a Bottle holder setup for now was a pretty simple hack saw job. The assembly stay nice an sold once tightened. Locktie probably RED lockite would be a good idea for an outside setup. This gives a simple setup for double bottle until I consider other other options. It also works with my legacy double bags behind the head approach.
The results of putting the seat on was a interesting.
I was too big for the medium; but just barely it pushed me just a tad too far forward on the bike and scrunched me up, not comfortable. Meanwhile the large is very comfortable. but a tad too big. Meanwhile Pluckyblond fits in the Medium perfectly. You can see that effect on how our head rest are mounted. (For the indoor setup I'm using the Thor prototype heat reset, for outdoors I plan to try the long proven ADEM's.
This is PB's medium. with the head rest at it's lowest stock position. She extend up and out of her seat.
This is my Large. The head rest arm had to be heated, straighten (not easy) and then mounted lower. Notice how very low. This is because the seat extended up past the point of contact with my back, (Still completely behind my body) So the headrest needs to be really low.
Because my seat is longer and mounted without the rear shim I have to rotate my bottole backward or they hit the head rest and the helmet. PB's bike they are almost vertical.
End result is that the Large Seat is more Areo than the Medium without the rear shim; my head remains just barely higher than my knees when pedaling. On the medium I'd be considerably higher and in the wind; or so it feels. PB meanwhile is higher now than on the stock seat but just barely, not enough to worry about it. In both case our forward facing angle is improved. The Giro helmets should work better and vision should be improved. From the trainers power feel easier to create, more time is required to prove that out.
You could get an equally low seat position on the medium if your remove the rear shim. However, that would then require bolting the back of the seat to the bike. Because without the shim the seat tips such that it only contacts the frame at the two mounting points; and thus the velcro provides no holding power.