So apparently when Francis Class told me that the problem with modern/vintage lasts is that they're not 'flat' he meant 'didn't have a heel' rather than 'as a board'. (So to speak.)
You see, it didn't quite sink in that, unlike maskmaking, when you take the last out of the shoe, you don't cut it out. So there needs to be a bit of wiggle room in the form of a toe and heel rise to allow you to slip the last out of the shoe without stretching and distorting it in the effort.
That makes perfect sense.
Thankfully, in the conversation I had with Francis last night, it turned out I'm not the first aspiring shoemaker to grab a piece of scrap pine and try to turn it into a last. He shared this link to the shoemaking blog Where Are the Elves? Adventures in Historical Shoemaking. This guy turned up an historical last to copy. It's a 'straight last' and I'm making a 'crooked last' but more on that another time.
Also, the sole is generally a bit smaller than the shadow of the shoe. So my last needed to look even less like a shoe and even more like a foot.
I can do that.
Tonight after work I grabbed my patternmaker's rasp (Hand tools FTW!) and spent some time with my last in a vise, re-shaping the sole and putting more of a narrow waist on the new last.
I might have to shape it a bit more but I didn't want to overdo it. It's easier to take off too little than to put too much back. . . or something like that.