Easiest way for the fight to be made.
Floyd sets the purses.
They fight on a UFC PPV event in Las Vegas. (Dana White is happy)
They fight in the octagon, but under boxing rules and regulations.
McGregor doesn't breach his contract with the UFC. The UFC widen thier brand through having Mayweather on thier bill. Mayweather makes $$$$$ and pisses of back into retirement.
You really think anybody except Floyd will be promoting this fight?
If it happens, everything will be on Floyd's terms and Conor will accept it because he'll still make far more than he usually does.
That's the top and bottom of it.
I really like the idea - but I don't see Mayweather allowing this to be promoted under the UFC banner, on a UFC event, with the UFC taking any slice of the pie.
This will have "TMT" plastered all over the PPV.
Alongside this, Floyd will want to use the PPV as an opportunity to showcase the fighters he is promoting on the undercard - I don't think the UFC will be keen to host a whole bunch of boxing matches on their show.