Duplicate Generic Wall and Change Width

I am trying to create a new instance of a Generic Wall Type so that I can keep all parameter settings of the original generic wall, but change the width.  My problem is the width for a wallType has no { Set; } (is read-only).  If someone can explain to me how to effectively create a custom Wall type with a specified width, it would be greatly appreciated.

using System;
using System.Collections.Generic;

using Autodesk.Revit.DB;
using Autodesk.Revit.UI;
using Autodesk.Revit.Attributes;



// Replace  a selected wall with a wall of different width. (create new type)
namespace RevitPlugin0816
{
    [Transaction(TransactionMode.Manual)]
    [Regeneration(RegenerationOption.Manual)]
    public class WallTypePrint : IExternalCommand {
        public Result Execute(ExternalCommandData commandData, ref string message, ElementSet elements) {
            UIApplication uiApp = commandData.Application;
            UIDocument uidoc = uiApp.ActiveUIDocument;
            Autodesk.Revit.ApplicationServices.Application app = uiApp.Application;
            Autodesk.Revit.DB.Document doc = uiApp.ActiveUIDocument.Document;
            Transaction trans = new Transaction(doc);

            trans.Start();
            List<WallType> oWallTypes = new List<WallType>();
            List<WallType> GenericWallTypes = new List<WallType>();
            oWallTypes = GetWallTypes(doc);
            //Get a list of all Generic Wall Types in the document
            foreach (WallType wt in oWallTypes) if (wt.Name.Contains("Generic")) GenericWallTypes.Add(wt); 
            //Duplicate Generic Wall Type: Generic - 8"
            WallType newWallType = GenericWallTypes[0].Duplicate("Custom WallType") as WallType;
            //locate parameter for wall width
            Parameter p = newWallType.get_Parameter(BuiltInParameter.WALL_ATTR_WIDTH_PARAM);
            //set the new width of the wall to 6.00
            p.Set(6.00);
            //ERROR: wall width is read-only
            trans.Commit();

            return Result.Succeeded;
        }//public Result Execute


        public static List<WallType> GetWallTypes(Autodesk.Revit.DB.Document doc) {
            List<WallType> oWallTypes = new List<WallType>();
            try {
                FilteredElementCollector collector
                    = new FilteredElementCollector(doc);

                FilteredElementIterator itor = collector
                    .OfClass(typeof(HostObjAttributes))
                    .GetElementIterator();

                // Reset the iterator
                itor.Reset();

                // Iterate through each family
                while (itor.MoveNext()) {
                    Autodesk.Revit.DB.HostObjAttributes oSystemFamilies =
                    itor.Current as Autodesk.Revit.DB.HostObjAttributes;

                    if (oSystemFamilies == null) continue;

                    // Get the family's category
                    Category oCategory = oSystemFamilies.Category;

                    // Process if the category is found
                    if (oCategory != null) {
                        if (oCategory.Name == "Walls") {
                            WallType oWallType = oSystemFamilies as WallType;
                            if (oWallType != null) oWallTypes.Add(oWallType);
                        }
                    }
                } //while itor.NextMove()

                return oWallTypes;
            }
            catch (Exception) {
                //MessageBox.Show( ex.Message );
                return oWallTypes = new List<WallType>();
            }
        } //GetWallTypes

    }
}