new to this
There okay buildings ,I would use liquid glue and a small paint brush be careful with all or any glue fumes. I do not no if any body makes a non-toxic glue for plastic.
a diagonal brace will go from the top of one leg to the bottem of the other leg on the same side of benchwork. I use 3 inch deck screws 3 on wide side of leg 2 on narrow end.