First get a union and cut a ring about 3/8 of an inch wide off the end of it. This can be made easier by putting the union on the end of a pipe (no, don't glue it on) and clamp down the pipe. I used a reciprocating saw for this, but a hack saw will work fine.
Wait an hour for the glue to set up some, go to the bench grinder and bevel the ring. If you don't have a bench grinder, use a rasp and finish it off with sand paper. If you don't have a rasp, go to the store and get one.
This is the end result. The barb can be tailored to fit the flexible tube exactly the way you want it to and it will hold. My solar heater is back online running just fine at less than 8 pounds of pressure. Here is a picture of the 'real' fitting that is needed. If you can find this thing, it will make your life easier, but none, that is exactly zero, of the places in town that I managed to contact had even heard of anything like this. If it wasn't for the price of gasoline I'd take this thing to each of the stores and show them that it exists and they should carry a couple of them. But, I noticed during this project that Home Depot doesn't even carry a 2 inch street ell in their store stock; I had to go to either Ace Hardware or Lowe's to get one. It's also nice to know that I can take this fitting and reuse it even though I cut it off the end of a pipe and it has a piece of 2 inch inside of it. All I have to do is use either an inside coupler or a pipe extender to hook it up. These two pieces of plumbing apparatus are basically unknown and not carried by stores. So, this project has bumped up my experience level and now, I pass it on to you.
Now, I promise I won't talk about fittings again until I get the flexible tubing in that I ordered.