Download and install TYPViewer from here, it is free. You will need to do this on Windows.
https://sites.google.com/site/sherco40/Start TYPViewer - change the language to English if necessary. Choose New from the File menu. Click the add button next to the Polylines box. Enter 018 in The Type box, enter 0 in the subtype box. The MPC Name should automatically come up STREAM.
Now the rest is up to you. Create a standard line by just entering the number of pixels width. If you want a solid color, check the No Border box. The other option is a Bitmap, which can be chosen from the dropdown menu. This lets you make fancy lines, like dashes or zigzags.
Double click the color box to choose a color, or just enter a hex value. Use a different color for night if you desire.
Checking the extended label box will give you some special options for size and color. They will be handled differently in Basecamp and different devices however, so do some testing if you want to use these. The No Label (invisible) option is useful when you want a line that never shows a label.
You will need to enter the FID of your mapset in the main window.
Now just save the .typ file and you're done. This program works directly with the binary file and you do not need to use cgpsmapper to compile it.
Now, inserting the .typ file into a mapset is another issue, and I think you are using a different workflow from me (I use Mapsettoolkit) so someone else will have to help you with that.
Javawa has a tool called TYPchanger that can be handy, it only runs on Windows since it uses gmaptool:
http://www.javawa.nl/typchanger_en.htmlUsing a similar technique, you can change the appearance of every object on the map with TYPViewer.