My vote for marking the screen is to use some screen other than an E-screen and scribe your own marks using a carefully placed straightedge and very sharp (e.g. Xacto) knife to scratch a single cut on the bottom surface of the screen. I've never done this for pano, but back in the day I did a decent number of screens for 8x10 crop marks. A single strong scribe mark done with a very sharp knife will be quite visible.
^ This.
I prefer the 645 format, so to help me compose I added lines to my Nikon's focusing screen. I tried a few methods: tape ("invisible" Sellotape, black electrical), fine indelible marker pen, fine pencil and a scalpel.
The two score lines made by a scalpel with a new blade were perfect. They didn't need to be deep. The other methods worked but were ugly and imperfect: the tape had an uneven edge (even when freshly cut with a scalpel), and tended to move slightly by shrinkage after applying; the pencil and pen lines where too thick and uneven.
The problem you're dealing with is magnification: looking through the viewfinder, the thinnest pen or pencil line appears to be several centimetres thick when overlain on the scenery!
This magnification also means the lines need to be placed extremely accurately: to the nearest 0.1 mm. The slightest error misplaces the line on the scene by several centimetres.
The method I decided on after trial and error and trashing a few screens:
Work out where your crop marks need to be, and do a trial run using low-tack masking tape, and test the screen in the camera. Repeat until the tape is placed accurately and both edges are parallel and identical (yes, it is a bit of a pain having to remove and refit the screen several times...!). Now tape your focusing screen to cardboard, itself taped firmly to a table or desk. Accurately make four cuts with a scalpel on the cardboard that align with the edges of your masking tape on the screen. Next, carefully and gently lift off these two pieces of tape with the scalpel, ensuring you don't move the screen even slightly. Then, using a very straight edge (I used a 6 in. steel rule) and the scalpel, lightly score two lines on the screen.
Tips:
- Do use a fresh scalpel blade
- 6 in. metal rule cheaply available from eBay
- Buy a couple of screens, one for practice that you'll be sacrificing
- The whole process is a million times easier if you use a desk magnifier to view your screen to do the above - again, easy to find cheap on eBay.