?
there’s a routier just before cannes on the national that runs parallel to the autoroute, they don’t open on sundays though.
out of respect its expected to buy a meal in the restaurant so not exactly free 
If you’re down there for an event at the Palais Des Festivals you will be able to park for free at Le Bocca. But be aware its only open whilst a show is on - as I found to my cost one year!
Hope this helps
Is the customs clearing place still there in Cannes?
If it is then it wasnt a bad place to park up. Mind you it was along time ago.
IIRC, head downhill into Cannes from the Autoroute then turn right on the main road / route heading back towards Nice. The central customs was around there somewhere, maybe near some railyards■■?