If you've already made both top and bottom parts of the mould, then you could use a narrow strip of sheet wax all the way around the lip of one half of the mould, assuming that there is enough vertical depth to do this? 

If so, the wider the wax strip the better, as this will give you the recessed lip in the part to bond the other half to and a thickness of 1mm should be about right, or as close to the expected part thickness as possible. 

This should work as a 'one-off' solution to getting a joining surface, but it will depend upon the part's depth being sufficient to allow for the joggled edge.

I already thought of something like extending the mould with a lip but I do not yet understand how to use the sheet wax in this context. When I use wax, I will not be able to use prepreg on it.