If you have miles with United you can use them to upgrade. If you got a last-minute fare, the chances are good that it's a fare on which you can use miles. Y or B fares are only 15K miles each way; M and H fares are 30K. As *G you have reasonably high priority on their upgrade list, so you'd have a good chance.

If you don't have any miles with United, perhaps you have a friend who does? If it only costs 30K r/t to upgrade, maybe you could work out a deal with your friend by gifting him or her a r/t ticket to the Caribbean, Mexico, Hawaii, whatever. Things like this might also be done on CC forum though I don't know how that works exactly.

You could also try looking at e-bay to see if you can buy a SWU, but I think most of those upgrades disappeared when United made their certificates electronic.

If none of these options works, I recommend a sleeping pill. Good luck with the flight, and have fun in BKK!