I wouldn't bother with a MOD, easier to just hand code an affiliates box.
Try the following code on your index_body.html file:
Code:
<table width="100%" cellspacing="0" class="tablebg">
<tr>
<td class="cat" colspan="2"><h4>Affiliates</h4></td>
</tr>
<tr>
<td class="row1"><img src="{T_THEME_PATH}/images/stats.png" alt="{L_STATISTICS}" /></td>
<td class="row1" width="100%" valign="middle">affiliate code here</td>
</tr>
</table>
</td>
<td width="18" style="background:url('{T_THEME_PATH}/images/mr.png');"></td>
</tr>
<tr>
<td width="18" height="18" align="right"><img src="{T_THEME_PATH}/images/bl.png" width="18" height="18" alt="" /></td>
<td height="18" style="background:url('{T_THEME_PATH}/images/bm.png');"></td>
<td width="18" height="18"><img src="{T_THEME_PATH}/images/br.png" width="18" height="18" alt="" /></td>
</tr>
</table>
Not tested.