Something you won't mind throwing out if you need to.
I usually hit the Dollar Store and pick up those Rubbermaid type
bowls (not brand name, though) You can probably get them
for about $2 for 3 bowls. And a cheap spatula for $1.
If you're lucky, it will all peel out when it cures.. but if it won't,
then you're only out a few dollars. Part of the cost.