I agree with the wet grass requirement, but might suggest foam wheels as they are a bit lighter than the rubber wheels. OR try this test. Get yourself some styrofoam or blue foam insulation (whatever is sturdy) and cut out some wheel patterns to find out the best size you may need. Heck, you might find these 'new' wheels will do the trick, very lite, and if you break one, you've got more styro/foam laying around.

Oh, almost forgot, you can use control rod to form the axle tube, maybe even small straws. Don't forget to glue on some hubs to reinforce the axle mount, when it's all said and done.

How's the weather you lucky son of a gun!?!?