@JimA The ski erg was simple once you have a Kickr, or any other bike erg which takes a cassette.
The key piece is a ‘ski cassette’ which slides onto the freehub and becomes the drivetrain. I think a picture tells it best.
The parts of the cassette are from lateral to medial:
Plastic, old cassette spacers, plastic, cassette spider piece, more spacers, plastic.
The plastic is just circles of plastic which fit onto the freehub. I used some plumbing parts.
Cassette spacers are just that off a broken cassette.
Plastic
Cassette spider piece- again off a broken cassette. This is the heart of the skierg. On one side tire a long piece of bungy cord. I believe I used 2x 4mm bungy. On the other side, tie some cord. I used some 3mm climbing tech cord. The tech cord pulls and turns the cassette. The bungy pulls and freewheels the freehub in reverse. 
More spacers
Plastic
The tech cord and bungy coil and uncoil around the freehub and are kept in position by the plastic pieces.
You need enough cord and bungy to get enough pull. I just redirected the bungy with pulleys of the Kickr base.
Above the Kickr, the tech cord connects to a rope that goes to a pulley on the ceiling and back down to climbing webbing tied as biathlon straps. You’d have to alter the rope bit to do classic style (a real skierg basically has a double freehub concept). I’m using double polling and am happy with that. I think the pain cave pic shows that.
If you build one, please share some pics.
edit for typos