For general optics you need a vertical line of light, e.g. a bulb behind a slit, not a thin beam. The way you get around this with a laser is to put a cylindrical lens horizontally in the path of the beam, so that it spreads vertically. In practice, the easy way to do this is to fix a bit of glass rod in front of the laser. Trouble is that it needs to be removable, since the A-Level lessons need an unmodified beam.
Step forward the two-part Agfacolor slide box, about 110 x 55 x 55mm, as sent out in their millions by Agfa in the seventies and eighties. They're strong rigid plastic, with a translucent lid, and at one end, on the outside, is a little niche designed to hold a label. I used one for the prototype, and it's exactly the right size and almost perfect for our needs. The label holder is just the right size to hold a little metal plate with a bit of glass glued to it, and there's enough free space inside for any additional electronics we might want to add at a later date.
Can I find anyone selling a similar component box? No. Which is where I hope people may be able to help. I've got three, I could do with at least 12 more. Anyone got any they don't want?