i was also wondering; are there any ratios for bed size to weir tube sizing? or any ratios for bed size and number of siphons in the bed? and randomly, can anyone advise me as to the use of PT lumber in building beds?
PT lumber in building beds is ok provided that you don't have any water making regular direct contact. Because the wood is treated chemically, water that makes regular direct contact may obsorb and leach the chemicals into your pond and possibley kill your fish. Many people build their frames with PT wood for strength & long lasting durability (and it looks nicer) and their bed boxes out of regular wood to ensure that this doesn't occur. Once your boxes are lined and sealed you should be good to go. On another note, if you have hard plastic beds like the picture below then this won't be an issue whatsoever.
I'm sorry I can't help you with your routing. I'm not too sure what the difference between tube weir and u bend is (and I'm a glorified plumber).
Thanks for responding, good advice! And, arent we all?
LOL. No, seriously... By trade I am a marine pipefitter (glorified plumber). I hope you can get answers to the rest of your questions :)