I would consider allowing a character with a high Con score to continue: i.e. one additional hour of crafting per Con modifier, but he must succeed in a Con check every hour, say 10 to start and it goes up ever hour by 2, failure means he looses the full day's work and 1/2 the gp and has to start over the next day.
intersting thought: something like a forced march effect on crafting?
fort save: (rolled in secret by dm)
dc= 20 + total level of spells to create + number days expended in extended crafting
pass: can do this the next day
fail: 8 hours of work only for this day and next. can try for extended crafting time the day after
fail by 5: snafu-must pay 25% of craft amount to fix
fail by 10: cursed, no noticeable effect
roll 1: fubar: srap the project and start frome scratch
take 20: not on your life!
take 10: no
roll 20: good day of crafting - no roll needed for next day: 16 hours of work for today and tomorrow.
something like that?
example: Heavy Steel Shield +5, ghost touch, arrow deflection
shield: 20 gp
mwk: 150 gp
enchantment: 100,000 gp
total cost: 100,170 gp
after the time and expense to make the mudane shield, there is a base cost of 100 K gp. cost to craft this 50, kgp giving a base of 50 days at 8 hours/day
eatherialness and shield need for special abilities, total spell levels = 10, so fast draft dc = 30
Not to be created by the weak or faint of heart!