Here's the instructions I followed with several modifications over the past few seasons. There are several other construction and design methods available - Condello has a great PVC design which utilizes 3' cheap mesh but still makes a 4' deep cage - this one just worked well for me as I can collapse the cages for storage during Winter. Let your imagination be your guide!
One challenge is sourcing 4' plastic mesh at big box store - if you order this from Stony Creek or other aquaculture sites I've checked it's pricey. You could probably get away with using 3' or 3.5' width which I've found at HD and Lowes - look for 3/8 or 1/2" mesh. Again, Condello cage employs the 3' mesh and still creates a 4' depth cage. The wider the mesh holes are, the less FA accumulates and the easier it is to clean when you pull the cages.
Few tips - Pool noodle floats are cheap and work well, but will break down over time and need to be replaced after several years. Not a huge deal, just something to bear in mind. Consider adding support section of pvc hose in the middle of the cage to help prevent the accordion effect. Zip ties seem to have a 3-4 year lifespan if exposed to significant UV so be prepared to replace them seasonally in various compromised locations. I still don't know if white or black have a better lifespan...I should be paying attention and recording that information, but have neglected to do so.
http://extension.umass.edu/aquaculture/s...eetJune2206.pdf 
