How do I place the seat back 20 cm with a full suspension bike? Here are 2 ideas for a makeshift design. The first one is no not that good. The second idea seems more doable using 2 heavy duty seatpost attached luggage racks.
The seatpost is 33mm diameter, dunno if that the official size but it's as close as I could measure it. The seat is great. The bike is aluminium alloy so I wish to keep it light.
Is there a pro solution to putting the seat back like this?
The only safe answer is to buy a bigger bike frame.
When your adaptor fails, you will descend rapidly onto the rear wheel.
The remaining original seatpost has an excellent chance of tearing your chest open, and impaling your head up through the jawbone.
While this is definitely creative, its absolutely not the right answer.
Buy a bigger bike frame.

If you're under 20, then there's a good chance you're still growing.  Even more reason to buy a larger frame bike.