I've made a few of these to see how I (and the bees) like them.
A separately screened 3/4" PVC pipe provides limited access for stimulative feeding in the Spring. A piece of screen is simply pushed into the normal (large) access area from the bottom.
The bees don't seem to mind and perhaps it avoids packing the brood area with syrup. In any case, it limits the number of bees that can "belly-up" to the trough. With some of my bees being 20 miles away, I know that the feeder will go longer before needing to be refilled... at least in the Spring.