I saw on Trail Tough's site the other day where he's making a bumper with a swing-away tire mount that also has a gerry can mount. Looks pretty good. I have a buddy that mounted a gas can on his tailgate as one of the other posts suggested, but he took it off because of the weight, also.
Here's mine, 31" spare and all. The can mount is by Con-Ferr, by far the best one I've ever seen. I got it at Dick Cepek in Phoenix. The can has been on there for about three years but I admit I don't use it often. I've had the full size spare for almost a year without any trouble.