You've got a lot more time to play around for the bonus level - you're collecting 99,999 pieces of gold compared to 2,500 or less in the other levels. As you can see, this means you have time to decorate. I started with the girders. The smiley-face cup is obviously there to increase productivity.
I'm particularly proud that every miner going down has no gold, and every one coming up has in this screenshot. A lot is to do with the one way catchements that you can see (where there's a branched tunnel backward underneath the main one). They're there because only two thirds of the miners will make it up a jump where the tunnel changes angle, and that's if it's perfectly constructed.
In the time it's taken me to write this I've gotten up to 46,000 gold. It's taken a few hours so far. I hope I win.