PCs have been able to buy custom magic items since the start of my DF Felltower campaign.
Here are the rules for enchantment speed: Enchantment Speed in my DF Game
The upside of that method is that it's very rules consistent with enchantment in GURPS in general.
The downside is that I need to know the energy cost of a magic item, then figure from that. Not a big step, or a complex one, but it is a extra step.
I've replaced it with a new approach:
Felltower Enchantment Speed
Divide the $ cost of all enchantments by 100. This is the number of days before the magic item is complete. Minimum is one day.
This is slower - a lot slower for very big items. But it does make things really, really simple. Instead of a 500 energy item costing $10,000 and taking 500/6 = 83.33 days, it's $10,000 / 100 = 100 days. Done.
It also pushes more in the direction I've been leaning more and more the whole game - find it, don't buy it. We also play a little less often thanks to my schedule and some unfortunate placements of holidays, plus occasional forays into Gamma Terra and AD&D. Net/net, it makes my life easier and that of the players, also, at a relatively minor cost to the characters. And really, if you absolutely need a magic item to be done now, there is always the option of a trip to Black Jans, a quick in-town based reaction roll*, and payment of double cost.
* So guys who don't hang around in town, or piled up a bunch of "free" disadvantages like Intolerance (Urbanites), Overconfidence, Odious Personal Habit, etc. don't have as much access to this as others do. And those who invested points in Reputation, Charisma, etc. or spend extra money in town get another benefit for doing so.