This ring was built during the summer of 2010 (published summer 2012) so we've had two years of use to work out the kinks. Issues we've run into are:
Water pooling - partially fixed by drilling holes in plywood and lining the holes with epoxy.
Delamination of treated plywood - minimized by addressing the water issues and caulking the seams between sheets.
Broken Corner post - Added angle braces.
Besides these items, the ring is holding up very well getting about 3hrs of work per week by 3-5 people... no heavyweights, though. If you're regularly going to be sparring with folks over 220lbs, you may want to consider 5/8" plywood.
Step 1: Materials List and Plans
Below is the materials list and the basic plans for the ring. Essentially, we're building a 20x20 deck with 4x4 posts every 10ft. The joists and bands break on these posts. The joists sit on ledger on the beams that cross the center of the ring and the band and these beams sit on 2x6 boards that are screwed and nailed to the posts and sit on the concrete footings that the posts are set in. Be sure that the corner posts will be high enough over the deck for the ropes.
5-6 treated 4x4x ~8' (length depends on how high off the ground the parts of the ring will be)
12 treated 2x10x10 (Band, center beam, scabs)
32 treated 2x6x10 (joists)
13 sheets of treated 1/2" 4x8 plywood
10 treated 2x4x10 (Ledger, corner braces, blocks between joists)
200ft of 3/4" twisted yellow polypropylene rope (found 300ft for $87 from Contractor's Rope, IL)
5lb box of galvanized 12d nails
5lb box 1 1/2" deck screws
10 4x1/3" lag screws and washers (to hold board to corner post that band sits on)
4 turn buckles
12 metal rings
~16' of chain
Lag hooks (to attach chain to corner posts)
1gal Thompsons water seal or something similar