I think we had added it following your instructions at http://www.mono-software.com/Mono/Pages/Discussion/dtopic/-oRV9mC-x0a65KLCAKcl4A/MONOX-doesn-t-seem-to-have-a-404-page/ (following up on a report our designer had done to this forum)
the source is:
</br>
</br><%@ Page
</br> Language="C#"
</br> MasterPageFile="/MonoX/MasterPages/Default.master"
</br> AutoEventWireup="true"
</br> Inherits="MonoSoftware.MonoX.BasePage"
</br> Theme="ClipFlair"
</br> Title="" %>
<%@ MasterType TypeName="MonoSoftware.MonoX.BaseMasterPage" %>
</br><%@ Register TagPrefix="MonoX" TagName="Editor" Src="/MonoX/ModuleGallery/MonoXHtmlEditor.ascx" %>
</br><%@ Register Assembly="MonoX" Namespace="MonoSoftware.MonoX" TagPrefix="portal" %>
</br><%@ Import Namespace="MonoSoftware.MonoX.Resources" %>
<asp:Content ID="Content1" ContentPlaceHolderID="cp" runat="server">
</br> <div class="container-highlighter" style="background-color:#666">
</br> <div class="container">
</br> <div class="row-fluid clearfix">
</br> <div class="span12">
</br> <portal:PortalWebPartZoneTableless ID="HighlightBanner" runat="server" Width="100%" ChromeTemplateFile="Standard.htm" HeaderText="HighlightBanner">
</br> <ZoneTemplate>
</br> <MonoX:Editor runat="server" ID="editor01" Title='<%$ Code: PageResources.Title_TopSection %>' DefaultDocumentTitle='<%$ Code: PageResources.Title_TopSection %>'>
</br> <DefaultContent>
</br> <p>Error 404</p>
</br> </DefaultContent>
</br> </MonoX:Editor>
</br> </ZoneTemplate>
</br> </portal:PortalWebPartZoneTableless>
</br> </div>
</br> </div>
</br> </div>
</br> </div>
</br> <div class="container">
</br> <div class="row-fluid">
</br> <div class="span12">
</br> <portal:PortalWebPartZone HeaderText="Left part zone" ID="leftWebPartZone" runat="server" Width="100%" ChromeTemplateFile="Standard.htm">
</br> <ZoneTemplate>
</br> <MonoX:Editor runat="server" ID="editor1" />
</br> </ZoneTemplate>
</br> </portal:PortalWebPartZone>
</br> </div>
</br> </div>
</br> </div>
</br></asp:Content>