The easiest thing to do is convert a kiddie trailer. I stripped off all of the old fadded nylon and bought some aluminium L brackets and a ceder fence post. cut cedar slats on a table saw and attached with stainless hardware. Nice looking flat bed trailer

My points of failure were when I tried to haul a 17 foot wood kayak and extended the tongue. Initial designs had far too much flex. Ended up using 1/2" emt. 1 foot parallel with wood separators.
eventual converted to a centered tongue and now tow it with a 15 speed tricycle.

The hitch is a caster with the wheel removed (from instructables) and a clevis pin.

If you lived near me I would reccomend that you pick up this used cargo trailer for $25:

https://sarasota.craigslist.org/bop/6169732740.html

It has what I consider to be a desirable feature...it can take BMX bike front wheels for replacements . I do not prefer the wheel chair type hubs.