What time is your flight home?
If its mid-late afternoon, consider catching an early water taxi, and then picking up a Belize Cruiseship Taxi Association guide for a little "in-country excursion" before heading to the airport?
It's going to cost you $25us for a cab anyway, so kick in a few dollars more and take a 3-4 hour detour!
Emmanuel Williams, 501 601 9261 or firstname.lastname@example.org would be one alternative, although I've never used him other than as a Taxi Driver, and just had his card in my Belize stack..
If you decide to try this route, be sure to leave a set of clean "dry" clothes handy to put on prior to boarding the plane, as the jungle does get humid!