What is a polite way to ask to 打包 (dǎbāo) "take away" my meal? Is 我想打包 acceptable?
No, just say 

"Hello! Waiter! I need (my food to be) taken away, is that ok?"
你好，服务员，我要打包，可以吗？


or

"Hello! Waiter! Can you help me package (my food)?"
你好，服务员，能帮我打包吗？


I don't know how you talk about this in English-speaking country, but I will use that sentence in China.

打包一下 is ok. This action only happened once, once it's done, you'll take away your food, the waiter knows about that. This putting your food into a bag action will not last very long.