Simple crafting during downtime?
The rules for crafting during downtime on page 187 of the PHB state:
You can craft nonmagical objects, including adventuring equipment and works of art. You must be proficient with tools related to the object you are trying to create (typically artisan's tools). You might also need access to special materials or locations necessary to create it. For example, someone proficient with smith's tools needs a forge in order to craft a sword or suit of armor.
For every day of downtime you spend crafting, you can craft one or more items with a total market value not exceeding 5 gp, and you must expend raw materials worth half the total market value...
Surely if you can craft weapons and armour you could craft ammunition as well.
Otherwise simply apply the same rules for crafting arrows that you've quoted from Xanathar's Guide to Everything (XGtE).
Modern Crossbow bolts (much like modern arrows) are typically made from carbon or aluminium but they can also be made from hard wood with some form of steel tip (again, much like arrows) so there's no reason the sames rules for arrows couldn't be applied to bolts (or a slightly modified version).
One thing to remember though is that D&D is not a simulation for real life. The specific materials, techniques or construction methods of the real world may not translate that well into TTRPG rules so don't get too bogged down thinking about it. There's no reason to reinvent the wheel; might as well just use the rules that are already provided.
XGtE even states on page 77 that:
The material in this chapter is meant to make your life easier. Ignore anything you find here that doesn't help you, and don't hesitate to customize the things that you do use. The game's rules exist to serve you and the games you run. As always, make them your own.