I know you've come to an answer already, but I bill this sort of project on a fixed amount basis according to the amount of time expended. I'll quote my daily rate and half day rate, and usually quote a discounted amount to enitice the client.
i.e (your rates will likely be higher

)
full day rate = $15 ( up to 8 hrs)
half day rate= $ 9 (0-4hrs)
If I think it will be a two hour gig, I would offer them a fixed rate of $7.
This eliminates all the charge for usage fees that most clients don't understand.