Bob, didn't you consider a vacuum board in order not to shade any paper area by the holding magnets and thus be more efficient paper-wise?
I cut a mat-board mask for the borders, too, so the paper will not have any bleed-over of exposed silver outside of the image area. Thanks again, Bob, you're a life saver!
Vacuum Board is a nice touch , I used them for large murals in the past and have them for small register prints.
If you had a standard setup for murals only (ie a wall enlarger setup) then a vacuum wall is preferable , but I would still set up the magnets the same way for positioning, and for very large sheets of paper the vacuum system must be sufficient to hold the paper flat without any added help which is a system beyond the scope of most darkroom workers.
Finding a good system that is movable as I use two different enlargers for Murals and smaller prints- 11x14 with drop table and 4x5 enlargers for my work is not practical in my current space as it would really limit me.
THerefore I have one metal board on wood which is easily moved from enlarger station and off depending on my work flow.
I never worry about the white border as I make matts for all my mural prints.
Just one more finikiy thing to worry about getting right when printing.
Did I mention painting the board black, this will help.
also all the space around the enlarger should be black and you should mask out the negative.
These steps will reduce flare.. the cut out mat-board is not a good idea as you will find flare on the edges from the white board, if you insist on the mat board mask go get Black board that when cut the core is black as well so as not to give you added flare.
Flat-bar will serve the same function as magnets if you're exposing vertically. All you need are two lengths. Lie them down along the horizontal dimension. Less of a hassle, easier to set up from print to print. I've never had a problem with holding the paper flat using this method, whether RC or fiber. Perfectly sharp, corner to corner with wide open apertures.
As for aligning the paper to the baseboard... I use a sheet of 3/4" mdf cut to the width of my paper. Just line up one of the horizontal edges and you're good to go.