You're absolutely right, Waldronate.

I managed with the lid still attached, but it was incredibly fiddly and took several goes. I used a magazine where you suggest pieces of card, because the magazine has a certain weight about it that isn't so heavy it will bow the glass and throw the focus out again, but its heavy enough to keep the map in position so you don't have to stand there for half an hour holding it still yourself as the scanner does its thing at 600 dpi.

I did a couple of scans at 300 dpi first, but even then they didn't pick up the finer detail of the pencil lines. I think the level of detail is also related to other scanner settings like contrast and brightness. You just have to play around with them till it looks right.