It is a subject frequently discussed and there is no concensus.
Some want to keep mundane craft as the only way to achieve high quality and truely artistic creation, other (like me) believe that if the mage is able to visualise clearly what he wants to do (which mechanically would mean achieving a good Finesse roll), he should have a reasonable chance to achieve it.
I believe RAW support the first trend, yet there are the rules for artistic creation in HoH:Societas (Jerbiton), so I have mixed feeling regarding RAW.
I started a thead some times ago, where I was proposing an alternative of a slower Rego magic allowing the magus a finer control on his magic, lowering the target Finesse. It is trying to achieve what you want through a different mean: I increase the duration of the Rego spell, thus increasing his magnitude to mimick the effect that instead of having a instant assembly, the assembly was done step by step (possibly requiring Concentration roll to sustain the effort). Each round of maintaining the spell was the equivalent of one day of work.
RAW does not support this approach, but I was finding it "right".