I've found some usefull information on using horizontal enlargers, and some just generally useful info at http://www.durst-pro-usa.com/library.html particularly #42: Making Big Prints in Small Rooms.
Well the plan has changed. I was going to buy the HK mural enlarger from a lab in NYC that was just looking to clear out some space. When the lab owner went to his storage, he couldn't find the head for the HK. Instead, he did find a Fotar 10x10 vertical 10' tall enlarger, with two color heads, that he's offered me instead. The horizontal elnarger appealed to me since I don't have extremely high cielings in my darkroom (a converted bedroom). However, I've found a few posts that say that the column of the Fotar can be cut down, and everything will work fine after you shorten the cables. So it looks like I'm gettting the fotar instead.
Now driving out of NYC with a 10' column in the back of my pickup is going to be interesting...
It's much better to use double-sided adhesive tape over the magnet! You do not need a metal wall, you do not have any prints of photographs of the magnet.
I routinely make prints 3mx1, 50m, adjusting the KAPA, laminating width Sallmetall 2m.MK-Czech Republic