The really simple thing, rather than figure out rules, is to:
1) get an idea what the player needs this construct to do
2) figure out a Resources/Contacts difficulty for hiring a mortal to do the same job, factoring in any complications you want to add to the construct
3) convert that to Thaumaturgy shifts
4) determine if the construct is always there, or "summoned" to deal with an intrusion: if always there, factor that duration into hiring DC, starting with a base duration of 1 scene; if summoned, add 2 to the summoning DC for each construct to make it a triggered effect, and *then* figure out duration.
The super simple (though expensive) means is to summon up enough shifts to kill the entity you want to create (26-36 or so), effectively summoning a character from scratch.