A new version of REnPortable is available on GitHub. This adds a line to the file \REnPortable\Data\RData\.R\Makevars so that rStan 2.18+ can be installed.
REnPortable is a portable 64-bit R environment for Windows. It was put together using the PortableApps.com launcher but is not endorsed by and does not comply with PortableApps.com requirements.
Please note that to use REnPortable you need to supply your own 64-bit copy of R (base), Rtools, RStudio Desktop (Open Source Edition). REnPortable was tested with R v3.5.1, Rtools v3.4, and RStudio Desktop Open Source v1.1.453 on a Windows 10 (x64) machine. No support is offered for any of these programs — please use the relevant support pages.
The reason for putting together REnPortable was to have a complete, portable R environment to learn R and RStan. Therefore, all the preliminary work is complete to install the RStan for Windows packages into REnPortable.
You can find REnPortable on GitHub for more information and to download.