Hi Koi, just as some additional explanation on why boolean union wasn't working for you - if you have a solid with "wall thickness" like this:



then it's not going to work to try and insert just one open surface across the top of it:



that won't make a volume because there would be no thickness in that top area. Instead of just one surface draped over the top if you formed an entire solid plug to boolean union in that could work but for restoring cut away areas it can be better to do "untrim" instead, there is some more info on untrim here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=444.4
http://moi3d.com/forum/index.php?webtag=MOI&msg=9671.5

- Michael