Well here's what I did.
Parts:
* Rear brake fluid reservoir from the BMW F650GS PN# 34 21 7 706 055
* 2 feet of vaccum line (Napa)
* 90° fitting is a 5/32" elbow connector PN#9873 (Napa)
* 1 M6 bolt (+ M6 washer)
Total cost: less than $40.
Notice the logo on the cap? Yup, Brembo!
Also notice the nipple on the cap is angled so it fits perfectly bolted to the ground bolt thingy on the side of the engine bay without bending the line.
I used a little under 2 feet of line just so I can unscrew/screw the cap without disconnecting anything.
I'll find out next weekend if this setup works as expected.