I originally read this thread when I was wanting to convert to kegs. Since then I sold all 14 glass carboys I had.
I LOVE fermenting in my kegs. I really wanted to ferment under pressure and force transfer. I purchased the parts and build spunding valves, but I only had the orange caps. They didn't do a very good job of holding pressure for my valve. I also didn't want to spend $60+ on a conversion kit.
Well the other day I set out to build something cheap...
I used a rubber stopper #7 with two holes and a large washer. I added a barb for the co2/spunding valve.
Here it is!
The build forum is here
I build it for a few dollars. And I only used a drill with a drill bit and a grinding stone...