This is how I did it, not necessarily how you should do it.
I'm not an engineer or anything so I cant recommend you copy me, but it worked out for me, up to yourself now.
http://500px.com/photo/5065476
This is underneath.
http://500px.com/photo/5065468
This is on top.
http://500px.com/photo/5065455
Hole bored in front of seat crossmember to allow access to bolt, nut etc.
http://500px.com/photo/5065444
Viewed from above this is the seat fully back showing crossmember.