One easy way I have used to lengthen several trailers is to cut the drawbar near the middle then either slip the next smaller sized stock tubing inside the cutoff ends or slide the next larger size tubing over them using a length of new stock that gets me to the length I'm looking for. Then drill and thru-bolt the two overlapping joints with at least three stout carriage bolts with nuts. I always try to overlap the original with the new stock by at least a foot, eighteen inches is even better. For a light weight trailer (for AI's, for instance), mild steel should be perfectly adequate and not very expensive. If you have a welder, you could weld it up instead of using bolts. If you hire a welder, the costs goes up dramatically. There are a lot of people who like aluminum trailers. I'm not one of them. They will, in my opinion, eventually fail, usually where steel meets aluminum, and dump your boat(s). A steel trailer, with minimal care, can last forever.
btw: any of you Aussies or your kin perhaps holiday in a little Southeast Asian paradise called Vietnam forty years or so ago? I envision a multinational fleet of AI's sailing Camron Bay to Paradise Island or up the Mekong...--a reunion of sorts.