I'd prefer putting one on the back, then throwing one up on the roof, as I don't like the look of it, don't warm to the idea of throwing one up there and use my roof for other things

I've got a mate who can dab a welder very well. But before we start planning on doing this, I'd absolutely love some ideas/opinions!
What to do and what not to do's.
Why have a swing arm over an arm that folds down (and vice versa).
Legalities of it all...
PICS are most welcome, as are personal DIY write ups in this thread!
I know most are probably on the Ls and MYs, but this doesn't matter, all are welcome.
Cheers!