Based on a design found here:
Modifications to original design: Now two parts, wheel and tire with a little notch so that the tire doesn't slip from the wheel.
Works with both 770 and 780 models, possibly others too.
Print wheel with a rigid material.
Print tire with a flexible material e.g. flexible PLA.
Slip tire onto wheel, the notches should give a good fit.
Find a metal rod which can be used as an axle (4.1 mm) and bore the wheel so it fits loosely.
If necessary: sand the tire a little bit down. I chucked it into an electric drill and with a moderate speed sanded it a little bit down.