1/4 inch would be fine for a whole sump, it's because of cost I use and recommend 1/8 for the sides. It also allows a little more water to fit in a tight place. If you use 1/8 for the lid, you will need to add the spines as pictured for my spray bar because 1/8 acrylic will warp and lose it's seal without extra support.
Yes Weld_On #3 for acrylic, just watch it wick all the way down the seam and it's welded into a single piece of acrylic water tight.
The best way to empty the res is to get a small powerhead from an aquarium store such as a Rio 90 (very small pump) and cheap too. run a hose through the lid and cap it off. when you want a res change just un cap it, plug it in, and pump it into a bucket. This creates less stress to the plants, and if you change it weekly it doesn't need to be scrubbed out until after harvest. I change my res weekly because it's small, water is cheap, and I don't have PH probs this way nor do I need to check EC PPM. If I start it at 5.3- 5.5 PH and do no adjusting of nutes for the week, it usually gets to 6.0-6.2 right before it's changed. IMO that's a nice PH swing and it's treated me well. Also this is only possible with R/O or Distilled water.
Root dammit root 
: The backup clones aren't rooted yet and I don't want any more delays so the new plan is to throw the mother plant in the aero machine when the clone gets to the screen... The mother is about screen height at the moment in soil under low wattage, We will see how this works out??? 
