This came to me as an epiphany. What you will need:
a. several magnets (I keep 8 on the easel. I started to use the magnets when I had a heavy burn on the edges to keep light from fogging under the easel blades. I found some small rare earth magnets at ACE that are very strong.),
b. some metal shim or plate stock and 2 small bolts with nuts (and some basic shop tools),
c. an opaque cover sized to cover all the easel blades at once (I use the heavy, black insert found with the Itoya album pages and it has a nice clean straight edge), and
d. a small incandescent penlight (the LED ones are too bright and get the ones that look like a tiny flashlight).
You need 4 small pieces of the plate stock to make 2 guides. One guide uses 2 pieces. 1 piece is straight and the other is bent into a L shape. An over-sized hole is drilled in both. The pair are assembled with the L on the easel blade and the straight piece overhanging the edge. When the nut/bolt are tightened, the straight piece should be lower by the thickness of an easel blade. The thickness of straight piece determines the border width. Mine where made from some scrap aluminum stock and it was easy to polish them to keep them from scratching the contacted surfaces.
Before removing the paper from the easel:
1. Two magnets are placed on the blade where the first border is to be made. They keep the blade in place for the next steps.
2. Place the two guides on the blade and secure them with a magnet on each one. The bottom of the L should be large enough to be held by the magnet. Be sure to check that the guide is seated against the blade edge.
3. Butt the cover sheet against the guides so it's parallel with the blade. Place more magnets on the cover near the border edge.
4. Carefully remove the guides and magnets. This should leave a thin gap between the cover and blade.
5. Holding the penlight about 4 inches above the paper, make 2 to 4 forward and back passes along the entire border. A black paper snoot helps to keep a narrow beam so the light doesn't pass under an adjacent blade and bleed-out in the corners. You'll have to experiment with the exposure to get to correct density.
6. Remove the cover magnets, then the cover and start the sequence on the next blade.
I know it sounds complicated, but with a little practice, it only takes a minute per side. I can crop as necessary and still have borders without filing the carriers.
As mentioned above this works only with 4-bladed easels but keep looking, they can be found at a reasonable price.