Attaching the new transom core

[bibijawa]

I've cut out the new 1 1/2" coosa transom core. I keep grinding here and there to get it to lay as close to the outside skin as possible but when I look down the top I can still see from 1/8" to 1/4" gaps in some places and I can't get it any closer. Is this normal or is it supposed to lay completely flat. I'm thinking when I apply a thick layer of thickened epoxy and clamp it all down all voids will be filled. Any suggestions or opinions on how to continue? I'm sorry I haven't posted any pictures but I just can't figure out how.