There is a far easier way to tighten that bolt under the seat then making a home made tool or removing the engine to get access to the underside, although it requires a little bit of feel and technique to fitouche the bolt back in place.
Get a thin wrench, something similar to what is called an ignition wrench. Place it under the head of the bolt and put a little upwards force on it to try and hold the nut up against the underside of the frame. This will give the nut a little bit of bite so it doesn't spin quite as loosely as it will without the upward force.
At the same time, use a drill driver with the appropriate sized nut driver at high speed to tighten up the bolt. If possible, put a little lateral force on the nut driver, again to help the nut bite into the underside of the frame.
The combination of high speed, with the small amount of bite on the nut will tighten the bolt back up.
This technique will work, just don't ask me how I know this.