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
Our seat was NEVER mounted correctly...
Our first ( restricted box stock ) was built by a conversion guy in Wisconsin and I fed off that general idea, and parts that came with new to me chassis as we advanced.