This set consists of 4 skids: The main skid that goes under the front fuselage, the rear fuselage skid and two skids to go under the wings. I highly recommend printing the skids with 50% or more infill and at least 1.2mm wall thickness. With this settings the main skid will survive 4-6 landings on a concrete strip. To make it strong enough to last "indefinite" just apply a single layer of glass fiber on the main skid and put a small metal plate on the bottom (see pictures). With this setup I have done more than 20 landings without much damage to it. The other 3 skids last long enough to just print a new one when the are gone. To avoid scratching the airplane surface I put a layer of tape on the contact surfaces of the skids.

... you can't see the skids but it does show a landing on a concrete strip without damage to the plane: