You will want to practice with scrap pieces of acrylic until you can bond acrylic with no bubbles and have the pieces exactly where you want them, before moving on to your refugium. Before assembling the pieces, fold the protective paper/plastic back about 1/2" from the bonding area and cut the paper/plastic off with a utility knife or razor blade. Insert 2 pins in the joint and tape the pieces together. Bond only joints which are flat. Do NOT attempt to bond vertical joints.
Here is an example of what your pieces should look like before applying the Weld-On. Note that the pieces are even on the ends and edges.