Making one should not be impossible, laminating one from marine plywood. covered with fiberglass cloth should not be hard. Others have made rudders for their larger O'Days that way. You might want to post an inquiry on hte Day Sailer Class Association web site,
www.daysailer.org maybe someone is parting out a DS II. A DS I CB might work, but you will need to change the pivot location (CB is adjusted different on the DS I vs DS II and III). It may be that the best option is to get one from Rudy though!
Where is Summerville? If you are within a resonable driving distance (one O'Day owner drove up fro mNew Jersey once!) to visit Rudy on a Saturday morning, you could at least save the shipping cost..... although it helps if you are "in the area" for more than just a visit to D&R to make cost of car trip worth it! If you did visit, call a day or so ahead to be sure that Rudy will be open that Saturday!
I've included the CB details from the DSA Handbook, unfortunately the DWG really shows a DSI CB, and I have never measured the location of the pivot hole in my DS II CB, if you have the top part of yours.... well that may not be a problem just copy the original.
Be sure to include the bushing that the CB actually pivots on when you re-install, otherwise the CB will not pivot smoothly since the bushing is slightly longer than the CB is thick and is clamped in by the pivot bolt through it. If your boat is a 1980 or newer, there is no bushing the CB pivots on a plastic pin that installs fro munder the boat. I can describe the process if you need it.