The only problem with that method is that the ray-casting still detects the surfaces, even though it's transparent. So you can't shoot through it.
To get a truly invisible wall that stops your physics base, but is invisible completely to ray-casting, you need to comment out the part of the .OPR that loads the object itself...after you've set up the collision object.
Here's one of my invisible wall .OPR files
OPRP
//~ LoadObject Editable Stationary Box24.box
AddCollision Default
CollisionType Box
CollisionBoxSize 7.216000 0.209000 4.384000
CollisionCenter -0.766000 0.646000 1.292000
CollisionTransformIndependently 0
CollisionMass 0
CollisionMassOffset 0.000000 0.000000 0.000000
CollisionFriction 1
CollisionBouncyness 0
CollisionBounceVelocity 0.1
CollisionGravityEnabled 1
CollisionDrag 1
CollisionBuoyancy -1
CollisionGroups 1
//~ CastShadow 0
//~ ReceiveShadows 0
//~ VisibleInReflections 0
//~ EnableLightMap 0
//~ CastLightMapShadow 0
//~ EditSurface Default
//~ SurfaceBlendMode Normal
//~ SurfaceBlendOperation Add
//~ SurfaceDiffuseMap
//~ SurfaceNormalMap
//~ SurfaceSpecularMap
//~ SurfaceDetailMap
//~ SurfaceShaderSpec
//~ SurfaceShaderEnabled 1
//~ SurfaceFlags
//~ SurfaceTextureFlags
//~ SurfaceSpecular 0
//~ SurfaceShinyness 128
//~ SurfaceDetailAmount 0
//~ SurfaceDetailMapScale 1
//~ SurfaceTextureAnimLayers 1
//~ SurfaceTextureScale 0 1.000000 1.000000
//~ SurfaceTextureOffset 0 0.000000 0.000000
//~ SurfaceTextureAnim 0 0.000000 0.000000
//~ SurfaceTextureStep 0 0.000000 0.000000
//~ SurfaceWaveParams 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000
//~ SurfaceNormalMapScale 1
//~ SurfaceDisplacement 0
//~ SurfaceNormalOffset 0
//~ SurfaceNormalMapIntensity 1
//~ SurfaceLuminosity 0
//~ SurfaceDiffuse 1
//~ SurfaceTransparency 0.61
//~ SurfaceTranslucency 0
//~ SurfaceColor 255 255 255
//~ SurfaceDrawOrder 0
//~ SurfaceNoDebugOverride 0
//~ SurfaceAlphaTestThreshold 127
//~ EditSurface Default
//~ SurfaceBlendMode Normal
//~ SurfaceBlendOperation Add
//~ SurfaceDiffuseMap
//~ SurfaceNormalMap
//~ SurfaceSpecularMap
//~ SurfaceDetailMap
//~ SurfaceShaderSpec
//~ SurfaceShaderEnabled 1
//~ SurfaceFlags
//~ SurfaceTextureFlags
//~ SurfaceSpecular 0
//~ SurfaceShinyness 128
//~ SurfaceDetailAmount 0
//~ SurfaceDetailMapScale 1
//~ SurfaceTextureAnimLayers 1
//~ SurfaceTextureScale 0 1.000000 1.000000
//~ SurfaceTextureOffset 0 0.000000 0.000000
//~ SurfaceTextureAnim 0 0.000000 0.000000
//~ SurfaceTextureStep 0 0.000000 0.000000
//~ SurfaceWaveParams 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000
//~ SurfaceNormalMapScale 1
//~ SurfaceDisplacement 0
//~ SurfaceNormalOffset 0
//~ SurfaceNormalMapIntensity 1
//~ SurfaceLuminosity 0
//~ SurfaceDiffuse 1
//~ SurfaceTransparency 0
//~ SurfaceTranslucency 0
//~ SurfaceColor 255 255 255
//~ SurfaceDrawOrder 0
//~ SurfaceNoDebugOverride 0
//~ SurfaceAlphaTestThreshold 127
Do NOT comment out the header: "OPRP"