Not sure I follow your 'no paper trail' point, and not sure of the importance ... unless concealing from spouse or significant other?
Using your remark, cash would need to be taken out at home - a paper trail of the withdrawal is created ... then convert using that service mentioned within this thread (Off St. Catherine and Peel) ... no trail there, but certainly one for a larger amount back home.
I use ATM - is $5.00 fee then like 2% of transacted funds for foreign transaction fee... use to not be there, but banks change all the time. Still MUCH cheaper than giving away USD$ to agency at rate of CAD$ .... that would not be wise.