Replace word tag to entire file content

Replace word tag to entire file content

Assume that we have a content xml-file:
<field name="id" id="1" type="number" default="" />
Assume that we have template file with tag:
INCLUDE_XML
We need to replace INCLUDE_XML tag to entire content from xml-file. We can
try.
tpl_content=$(<tpl.xml)
xml_content=$(<cnt.xml)
xml_content="$(echo "$tpl_content" | sed "s/INCLUDE_XML/"$xml_content"/g")"
echo "$xml_content" > out.xml
The problem is unterminated 's' command cause xml-file has lot of bless
characters (quotes, slashes, etc). How we can do the replacement without
this care about the characters in content xml-file?