Car trailers

[Jack91]

Hi all,
Has anyone built and regod their own 3t agm car trailer? Im a boily by trade so the making of it isn't an issue, but is it hard to actually get it registered? Id probably buy plans off the net.
I just cant see $5k + to buy one, considering its probably 2 in materials, probably a lot less considering what offcuts i have access to for free.
Im only after a basic skeleton type to keep unnecessary weight down.
Thoughts? Worth doing or big headache with registration?