container.setWidth(maxWidth + 2 * X_SPACING);
int x = (int) Math.round(maxWidth / 2. - SLOT / 2.) + 1;
int y = 17; // initial space for label
container.addBoundary(Boundaries.BACKPACK);
// backpack slot
container.addSlot(new SlotBackpackOnly(inventorySlot, 0, x, y));
container.addBoundary(Boundaries.BACKPACK_END);