So this is a project I've been researching and gathering parts for the last several weeks, and it's now ready to show off. I realize this tether management system is not for everyone, as it does take up quite a bit of real estate, but for being put on a boat, I think it's perfect and just what I need to keep the tether neat and tidy and have a place to mount the topside adapter. Please ignore the rest of the ROV parts and tools all over the table, I have a few things I'm working on.
The cable reel I used is an Alert Stamping 700WR Pro-Reel Cord Carrier ($27) and can be found here: http://www.amazon.com/Alert-Stamping-7000WR-Pro-Reel-Carrier/dp/B0000C6E3P/
The slip ring is made by Nextrox and has 6 conductors ($11), and it can be found here: http://www.amazon.com/Nextrox%C2%AE-Wires-Conductors-Capsule-250Rpm/dp/B00CE3S74C/
That's basically it! The only part I had to make was a 4" disk made out of a sheet of Plastruct and spray painted a gloss black, but you can really use any material you like to mount the slip ring on and mount it onto the inner reel. The yellow/green wires coming from the slip ring must be glued (I used clear marine silicone vice hot glue) to the frame so it does not rotate.
The topside adapter is velcroed to the grey frame for easy removal.
Let me know if you have any questions, I have some more pictures of the build, but I didn't want to clutter the board.