I would like to add an ash drawer with a slot. Where does the slot need to be placed? Does such a thing make sense at all?

You can install an ash drawer in the front between the front panel and the bottom panels. However, it is not absolutely necessary, because the wood-burning ovens are usually so efficient with good insulation, that even after 4 to 6 hours of firing, a maximum of a handful of ash is produced. The easiest way is to sweep out the oven when it is put into operation, as it is usually cooled down.