This bird feeder works great as is, however some modifications can be made to improve it even further as listed below.
a. If you have a deep tray ( which is best for holding lots of bird seed ) you might want to drill out drain holes along the perimeter of the tray such that when it rains water will not pool and collect in the feeder tray. Make the holes small enough such that the bird seed will not go through them.
b. You can easily attach a tube feeder to the top of this bird feeder, I have done it with two. I currently use droll yankee feeders which have mounting points on the base of the tube feeder. If you have something similar to this simply place the tube feeder in the center of the tray, mark the anchor holes with a sharpy and drill through the top base cap. You can then anchor the tube feeder to the top tray, through the baffle cap and into the top support. This is a great way to double your seed capacity, and gives the smaller birds some seed to get to when the blue jays are ravaging what is in the tray.
Good luck with this project, it has worked great for me and I hope it works for you too! It is made from easily accessible parts had at any hardware store.
If you need further help building this feeder, take a look at the accompanying video here:
Part 1: http://www.youtube.com/watch?v=3Qdm747cols
Part 2: http://www.youtube.com/watch?v=xxdxdck4Syo