I installed in shed.  16" spacing between 2x4s rail.  Product does not address multiple rail use.  Needs a tie together between rails part.  I bought some cheap U-bolts and used the flat piece with some cheap bolt hardware to address this.  No the rails are all even.  90 corner piece would be nice.  Shorts rails would be nice too.  The black cover slides on the steel no way to lock its position once installed.  The variety of mount is limited compared to other makers.  LIked the fact it had a steel under beam. Cost to much for what it is.