Best option? Get an old knackered caravan, rip body off. This leaves you wth a good (if its galvanised) chassis, plenty of extra metal to make a framework, the suspension (the most expensive single items to buy if you want to make a trailer from scratch) the hitch, possibly brakes, all the electrics.
And it gets one more caravan off the road
Making a trailer is fairly easy. Just go to a local Halfords, look at a small Erde type trailer. Anything you make WILL be stronger than one of those, so as long as you feel its strong enough, it should be. Good old common sense is a good indicator of if something is strong enough. Remember, YOU are loading it, YOU are pulling it, so if YOU are happy with it then it should be OK.
And it depends what you are after, a trailer for putting some stuff in to take to the tip? A nice looking one for taking on holiday? A heavy duty one to carry a car? Just decide what you want and try and copy an existing one.
There are currently no laws on home made trailers, but that will change, but probably not for the next few years.