Hey JDM- Thanks for the comments- Feel free to copy whatever you like for your personal use.
When I join the tubes to the base, I use a custom ground fly cutter to cut a 1/8" groove in the base, then I use Weldon #4 to glue the tube in to it. This gives it a much stronger joint, and looks very clean. For this design, the small tube goes all the way though the base though- For that one just cut the hole so it is snug, and glue with Weldon #4. Follow up on the inside with a little Weldon #16 if your hole was a little too big.
I use 1/8" O-rings all the way around. I use a similar fly-cutter, ground to a 1/8" wide flat bottom profile, to cut 0.1" deep grooves for the O-rings to sit in. Keeps them from getting in the way, and looks professional. (On another thread I posted a picture a few days ago, but I forget which one.)
As for the drill/tap of a tube- I don't like doing that at all. It is hard to do, and prone to breakage. The joint itself is very weak, and if you hang a pump or long pipe off of it (lever arm), you are just screaming for a leak in the future. For my designs I refuse to penetrate a wall of a tube. For home use, it can be fine, but for commercial use.... too much of a chance for damage/leakage. (Note- I've repair some commercial units that do that, and they always start leaking again.) I'd suggest that you see if you can come up with a way that doesn't require the penetration.
Re: the swirling- I'm not convinced that it makes a big difference to the foam production. Think about it this way- There is a set # of gallons per hour going in, the same amount coming out, and the skimmer can only hold so many gallons, regardless of how much the water swirls. By swirling the water, you force some of the water to get dizzy at the top of the skimmer, while some is forced right out the exit. I'm not sure the swirling doesn't create extra turbulence, which can hurt foam production... I'd like to see a test with a skimmer that swirls, then remove the swirl mechanism to see if the foam production (over several weeks) changes at last. I'm not convinced that swirling is bad, but I'm not convinced that it will buy you anything either.
By all means make yours swirl if you desire- I'd like to see the results.
Zeph