06 Multichannel Stems.RPP
08 Multichannel Stems.RPP
10 Multichannel Stems.RPP
12 Multichannel Stems.RPP

Here's templates and render settings

To make a custom multichannel just add or remove tracks, set the number of tracks on the Multichannel track ROUTE and if you add a track create a Receive.
Select the Multichannel track to render along with time selection.

Attached Files (Click to download or enlarge) (Only available when you are logged in)
Reaper-Multichannel-Render-Settings.png (270.79 KB, 42 downloads)
Reaper-Multitrack-Sends.png (40.55 KB, 42 downloads)