There are two ways to do this. Add "spawnflags 6" to the model entity, or put playerclip around it. The spawnflags method will cause q3map2 to turn the model into metasurfaces for the bsp(effectively making it a part of the map's geometry), and allows it to be lightmapped and solid. The playerclip method makes the model solid, but can't be lightmapped. The playerclip method is much MUCH faster if your model has a lot of polys, though. It's speed is at the cost of geometric accuracy, though.