You can self build trailer so long as you make sure they comply with lighting and load regs
I think if the overall with is less than 4' then you can use a 3' light board. Over 4' and you have to use a 4' board as minimum with nothing protruding more than 18". That means you have to ensure your lights are reasonably close to the edge as you can get them.
Load wise if total gross weight exceeds 750kg then you have to have braked wheels. Under 750kg then you don't need them.
There's a few more requirements about dimensions and weights which
this article lays it out in quite nicely.