It is a very simple trick I learned here. The Tamiya plastic cuts like butter with thread. Basically you drill a small hole in the molded in hatch line. Thread a piece of thread through this. You then just start a sawing action with the thread, going back and forth and cut along the recessed hatch line. Cut around the hinges, and then all you have to do is drill a hole through the hinge assembly and push a piece of brass rod into, and you will have a fully functional hatch.

The thread will break a few times, but you just thread through a new piece and finish where you left off. I bet it took about 10 minutes for each hatch, tops. On the Heng Long it takes considerably longer.

Here is a shot of my Heng Long Pershing with the hatches opened.