Hey guys: Geezer here. Tinker asked about the proper way to hold a medieval arbalest. The good news is: there IS no proper way. Medieval crossbows range from little pistol bows to big siege bows, but generally, there are four sizes. First, you have the short 'carbine' bows of the 16th century. These are generally about 24-25 inches long and were mostly shot 'free-hand' That's held with the butt alongside the cheek, braced right at the very tail with the 'trigger' hand. Remember, on most medieval bows, the foresight is the point of your bolt. For long-range shooting, the free-hand shooter maintains a constant aim over the point, while lowering the butt of the stock to the necessary elevation. And of course, recoil isn't much of a problem anyway... at least not until you get to Really Powerful bows.
Next come the 'field bows', like the Maximilian I and Louis XII bows in the Kunsthistorisches Museum in Vienna. (looks like most of the crossbows in the museum at Malta are the same size) These are about 30 inches long. These are sporting or military bows you might carry into the field as a personal weapon. A 30 inch stock is short enough to hold free-hand, or you can rest the butt either atop or beneath the shoulder, depending on the range.
Then there are large military bows... they're generally about 36-40 inches long, still 'field' bows, but pretty large and a bit unwieldy. Those are probably too large to shoot freehand, so should be braced atop or beneath the shoulder.
Last come the siege bows, some approaching 6 feet in length and similar width. Siege bows are too big and heavy to shoot from the shoulder. You'd want to brace them on a pivot, or at least put 'em on a wall or something.
So having said that, I'll admit I mostly shoot with the narrow butt of my favorite target bow atop my collar-bone (clavicle) and against the trapezius-muscle. There's a nice little pocket that lets me brace the bow securely, yet carry it high enough that I don't have trouble getting my eye down to the top of the stock.
There you have it: According to me, there's no right way to hold a medieval crossbows, though there are better and worse ways. And yeah, medieval crossbow butts are often quite narrow. You have plenty of wood to make a stock... or add some more to the bottom, put some fancy inlay along the joint and call it a feature. That's a lotta fun too. So what are you waiting for? Get to it. Geezer