I don't really have an answer for you but I do have some things to consider/questions.
Do you expect to do this inside or in the parking lot?
What type of tuning are you considering? Just waxing or touching up edges too.
If waxing, will you have power for an iron or will you just be rubbing on?
If waxing will you be brushing or using roto-brushes?
I'm asking these questions because the sturdiness requirements of the portable bench change based on the force you're going to apply. When scraping, rubbing and (hand) brushing, it is best to have a very sturdy bench that doesn't wiggle while you're trying to work. Scraping and rubbing don't require (at least for me) the sturdiness that brushing does, but if you're using a roto-brush, that goes away.
The bench in post # 44 would be much sturdier used in the snow than a parking lot because you could secure the feet.
I like the one that attaches to the bumper but like Jacques stated, it is much too low, unless you are good with working on your knees. I'm normal size (5'7") and that would kill my back. Maybe find one of those height adjusters they have for hitches and see if you can bring it up some. It would lose some rigidness though.
I've also have seen trailer hitch bike racks that could probably easily modified.
Another thing to consider is making sure your tools are clean and sharp to lessen any forces you need to use (i.e. scraping) and have your skis in perfect condition ahead of time to minimize the amount of touching up you might need to do.
For rubbing on wax, I like this:
Works very well but it is difficult to get hard wax done correctly. It's Ray's Way WaxWhizard tool. I've found I can just rub on and brush off. Not as good as hot waxing but it good for a day of skiing.
Have fun,
Ken