I don't think its proper advice to recommend mounting an aluminum seat with a 5 point harness the same way you would mount a regular kart seat.
Though, the original post doesn't define which type of cage kart he has
Outlaw kart uses a front mount that is essentially an angle aluminum mount that mounts to the kart and can pivot slightly.. on the rear, there is a seat mounting hoop that bolts to the chassis and the seat bolts to it
QRC should show their stuff.. it will adapt to most chassis.
RFC has a good setup too..
Look up Blackhawk Chassis on facebook.. he will build what you need, and it will be good
I looked all over. And I couldn't find anything for our outlaw kart seat mounts..
Here's one thats a hoop.. this is a QRC part. Im sure... sorry its a crappy picture
There are two "wings" that goes from this mount. To the seat... they mount with rubber biscuits on the connecting points