Use this with an

// "Invisible" texture script
// Cross Lament claims no credit for this
// She also refers to herself in the third person

default
{
state_entry()
{
llSetPrimitiveParams( [ PRIM_TEXTURE, ALL_SIDES, "b8d3965a-ad78-bf43-699b-bff8eca6c975", ZERO_VECTOR, < 0.4, 0.4, 0 >, 0.0 ] ) ;
}
}

alpha texture: