I've given this airtrap some thought, and came up with the one you know:
http://forums.procooling.com/vbb/sho...&threadid=5613
with 3/4" PVC parts, it really isn't flow restrictive at all, and clears air in about 10 seconds, even in the smaller size.
Everything is optional: if you don't want a hole in the top of your case, that's fine. If you don't want clear tubing, use normal white PVC. If you don't want the interchangeable barbs, just go straight for the 1/2 NPT connections, teflon/screw in your polypropylene (grey) barbs, and forget about it. It's all non-metallic, so not reactive with anything.
The cap (with Teflon) is easily removeable, and makes an excellent seal.
Also, if you really don't want the U shape, then just use the tee, and connect a barb horizontally.
I'll be hooking up my flowmeter to it, which will screw right into it.
I'll also be adding a filter:
http://forums.procooling.com/vbb/sho...&threadid=7263
It'll fit perfectly, as there's a ring inside the fitting to hold it. A simple tab will allow me to pull out the filter for cleaning.
Now I'm sure that if I gave it more thought, I could come up with a better solution. The trick really, is to ideally make the water travel horizontally, over a wide area, to allow the air to bubble up, with a relatively slow flow speed (not flow rate), or over a relatively long length.