Making your own chassis just got harder here in Oz.
You will have to specify and calculate the strength of the drawbar, or test it.
For example, a "very small trailer" is one that weighs max 750kg loaded and doesn't need brakes.
You will have to either calculate or prove empirically the draw bay will stand longitudinal tension and compression of 1.5 x 9.81 x 750 = 11 metric tonnes.
It will also have to withstand transverse, upwards and downwards thrust of 0.5 x 9.81 x 750 = 3.7 metric tonnes.
To put it in American/English terms, if you build a trailer that will max out (trailer plus load) at 1650 pounds, you will have to prove, by way of approved calculations or actual test work, that the drawbar does not stretch or bend under 24,200 lbs of longitudinal tension or compression, and under 8093 lbs of transverse or vertical up/down thrust.