G&P makes a full length RAS II that might be what your looking for. It's as slim as a regular RIS but with the top spine rail you seek. I don't know how fat the SIR is, so I'm not sure how they compare. Here it is at WGC:
http://www.wgcshop.com/pcart/shopper...IS%20and%20RAS
And installed (RS):
http://www.quarterbore.com/kac/kacrasii.html
HTH