Man - tough deadline there guy...

OK try this....

one of the major barriers to a successful boolean union is objects which are not solid prior to joining them.

1) Make sure there are no obvious holes anywhere in your geometry.

2) BEFORE you mirror the object take the center cross-section and apply a surface to it right down the middle.

2A) This is sometimes difficult, and if so select the curves completely around the cross-section making certain you don't miss any tiny curve portions.

2B) Now that all curves pertaining to the cross-section have been selected hit "copy."

2C) Select everything and choose "hide."

2D) Hit "paste" and you should only see your 2 dimensional cross section edges.

2E) Hit "join"

2F) Position your viewport angle so that you are seeing the object from the side (not as a single line)

2G) Draw a line with object snapping on along the object shape from left to right or top to bottom along the longest portion of the object and snapping to the curves on each respective end of the object.

2H) Select the joined object and then "trim" it selecting the line as the trimming object.

2I) Deselect all. Then select the line and hit "sweep" choosing one of the two sections you just created by the trim action as the sweeping rail. Deselect all, then repeat using the second trimmed section as the sweeping rail.

2J) You should now have two planes which meet along a line but are separate objects. Select the two planes and then hit select "inverse" and delete all the lines and curves.

3) Hit "show" and choose all so that everything shows.

4) BEFORE you hit "mirror" this time select the half you intend to mirror along with the two new cross-section planes you just created and hit boolean "union."

5) Mirror the new solid object and then boolean both halves together.

Hope that helped.... next time it would be a lot easier when under a deadline like that to place a cutaway of the non-mirroring surface here in 3dm format and one of us could just do it fast for you and explain it all after your deadline. ;)

Good luck, I hope that helped!

- 3dvisuals dude