Opinions vary on the Weld-on. For a small thing like an overflow box I'd use Weld-on #4 It dries fast but not as fast as #3 which is insanely fast. For larger items I use Weld-On #40, as it gives you more time to apply and is a thicker substance. Others use Weld-On #16 which comes in a caulk type of tube. #4 and #40 need a seperate container to apply.
Here is the link to the different types of Weld-on
http://www.ipscorp.com/ind_html/acrylics.html
And here is the Selection guide
http://www.ipscorp.com/ind_html/selectind.html
Where to find them at? I get my supply at a local dealer. Look in your phone book under "Plastics-Rods, Tubes, Sheets, Etc" as Weld-On is considered a Hazardous Material and a special Hazmat license is needed to ship it in the mail. Eaiser to just go local.
THe Siphon TUbes are a pain in the rear. Jeff at Life-Reef sells them although I've never bought them from him. Also I just gound Aquatic-EcoSystems sells them but again I've never bought them. THey also sell the Acrylic tubing if you are going to buy some and make it yourself.
Let me know if I can be of any further help. Also ask broncofish as he's got a pretty extensive hands on knowledge on building acrylic boxes of all sorts.
