I wouldn't advise that. You're likely to get an impression of the tea towel on the litho and also you're cardback may end up smelling of baked beans and who's going to take Darth Vader seriously when he has the whiff of Heinz about him?
If you don't mind the wait, just place some heavy books across the entirety of the card (perhaps not on top of the bubble itself) and leave it. For weeks. You'll have a perfectly flat card when you come back.
Some say that placing in an acrylic case works. I find it doesn't and it's also going to end in tears unless you wrap the card edge in paper as you insert it. Reason being the case will only bend it as far as it absolutely has to in order to fit the edges of the card under the acrylic rails. Placing a cardback in, and subsequently removing it, it likely to drag the litho across the sharp edge of these acrylic rails and cause damage. I learnt the hard way
Now I just wrap a piece of a4 paper around the edge of the card when I insert it, almost regardless of how flat it lies. In actual fact, a Star case is more likely to flatten a card properly than an acrylic case, IMO. That all said: go with the books idea. Can't fault the results.
