I use a several step process. The first step was to make a casting in Hydroshrink. There are a lot of 1/87 modelers who have been shrinking vehicles from 1/48 and larger scales, see http://87thscale.proboards.com/index.cgi?. This can give you about 66% reduction. To fine tune the remaining scale difference, I then make a resin part replica of my 66% reduction, fix any distortion issues, and make an RTV mold with Naphtha mixed in. The Naphtha will evaporate out of the RTV rubber over about 5 days, shrinking the whole mold in proportion to the amount of Naphtha used. I have found that you need to get the RTV/Naphtha soup to cure as quickly as possible, otherwise the Naphtha will start to settle to the bottom, causing warpage. I have found a catalyst from Smooth-on for their tin-cure RTV, but you need an accurate scale to weigh the proportions. This process is how you see some strange N scale offerings, like that '74 AMC Hornet.