Receiver tube is special in that it does not have an internal 'seam' where the tubing was bent and welded together. Go to your steel yard and ask for receiver tube, they'll know what you are talking about.
If you buy 2-1/2 with 0.25" wall square tube, the dimensions work, but the seam will block putting in a tube.
Also, if you make your own receiver, put some sort of flange on the end or you could 'flare' the end where the hitch goes. I just welded some 1"x1/4" flat bar all the way around and it seems to be holding up well (probably 2,000+ miles so far on the trailer).