I have been welding plastic for years ( I work in a body shop). The first step is identifying the type of plastic ( using some of the original tub shaved off the edge works well if you dont have plastic rods). A soldering iron will work if you don't have a hot air or nitrogen plastic welder. Clean it really well, with thinner, or alcohol, no need to scuff since you are melting it together. If you are using a soldering iron, I always stitch it first ( push the hot iron into the joint melting both sides almost all the way thru, almost overlapping each time) then I go back with my welding rod and fill in the craters made by stitching and kind of flow it all together That way you get a weld almost all the way thru the tub from one side. then flip it and do the other side, now you have a weld all the way thru. As far as holding the joint together, I use a hot stapler, but aluminum duct tape on the opposite side you are working on works well.
