Well, there are a couple of ways to do it.
1. Make your pour mold with a very small slit in the back. (You MUST use silicone for this, otherwise flexing the mold to get the lures out will crack the mold) Then over-fill the mold when you pour and use scissors to trim that extra off when the lure hardens.
2. Make a 2-part mold with a pour spout, just like a lead mold. This works very well, but is slower than method #1.
I personally find that my lures don't mind having a flat back.
