Trailer building shops usually have rectangular tube. 2x4 is common, 2x3 is not common. What wall thickness, Can you live with steel and if so, does it have to be galvanized? It is not good to weld on galvanized, you have to grind off the zinc and then redip it. There are some paint-on Galvanizing which are really good.
Give us some followup.

I,ve got a peice of 2+3 steel .120 thick. I can lopp off a 8" peice and send it to ya.

Call me and I will send you what you need.
Sorry for the delay. It's been really busy and on te road.
Need a day on the water!!!!!!!