Fiberglass is a viable option. Aerospace companies like INFINITY Aerospace, http://www.infinityaerospace.com/infgear.htm , are manufacturing fiberglass main gaer for full size aircraft. I have not seen these for RC applications yet but they may be out there, your budget might limit this option though... You could add multiple layers of glass in a quasi-isotropic layup to the belly of your aircraft but it will eventually fail as it gets warn away on multiple landings; especially on a 10+ pound aircraft. A grass field would help with short term erosion issues. Would a single wheel mounted to the belly, similar to those used on gliders, be possible? If so you would need a simple skid on the wing tips to protect them on landing.

Mechanical retracts would be the easiest retractable option. You would need to reinforce the wing to accommodate the gear and the will add about 10 oz with the wheels and retract servo installed. Another option might be to mount fixed gear on the wing outboard of the fuselage. A third option could be to mount a manufactured, non-metallic, gear such as DuBro's or Goldberg's, to the fuselage.