Personally, I wouldn't even bother with anything special; I would just stand the roll on one end in the darkroom, wind off about as much as I want to cut (roll it up loosely by hand) and hack it off with a scissors. Right away, put the large roll back in the light-proof bag and tape it shut (If you don't do this, keep saying to yourself, over and over, "the big roll is out," otherwise sooner or later you'll forget about it and turn on the lights). Then lay the loose roll down on its side and feed it into your paper trimmer. Trim the first end "square," then cut the rest to size. Save the loose ends for test strips if you want.
When you are cutting down the loose roll you spooled off, avoid letting it rub across the countertop. Rather let it roll by itself. When it comes up close to the trimmer, pick up the loose roll part and unwind more while setting the main loop farther away. Make sure that your countertop is smooth and clean before starting. Make sure your hands are clean, too, and avoid touching the emulsion side. If you tend to make finger marks on the emulsion, you may want to wear one of those light-weight white gloves on one hand, or at least use a small pieces of paper to hold down the part you are trimming off.
When I worked in QC at a large chain outfit, we would sometimes cut off perhaps 50 narrow strips for sensitometric wedges for some test or another. For this we'd use a crude wood fixture something like your first drawing. But practical experience says don't make holes; it's too awkward to load. Rather, just make some "V" notches in the uprights, and let the weight of the roll hold it there. Also, put some sort of collars on your shaft - if one end slides out your main roll will drop and possibly put a "dent" in the roll, likely leaving pressure marks through a number of wraps.
The light-tight bags are probably more rugged than you think. In our outfit we used to slit a lot of our own paper, and we used the same bags - brown paper outsides with foil liners - over and over. Tears on the outside were covered with black masking tape and all were checked for light leaks before reuse, but we would see your use as light duty for a bag.
Maybe I should say that I've handled a large number of rolls, so I'm comfortable with this sort of thing. If you find that you're awkward with it, or can't stop worrying, etc., you just need to find a method that works for you. Note, be very careful not to get a fingertip into the cutting zone of your paper trimmer. Best of luck.
