Hey Tews19!
It doesn't take much to hold a seat, just look at the originals..
I haven't done that specifically but it would work assuming the seat pad was somehow installed over and removable so you can get to the screw heads. The option you linked is great if the nuts are tack welded to the struts. But then again, with either machine screws or bolts, you could skip welding struts in place. Instead, epoxy the screws/bolts so they are firmly fastened to the seatbase and then you have just the threaded portion protruding below. If the screws/bolts are fixed, then the nuts obviously are not. That's OK, just do not over tighten the nuts so you break the screws/bolts free. You could even use wing nuts if you think it would help servicing.
The Honda 4s all have a sturdy hinge setup, so why not use that? It makes some service work easier. One easy way is to take some tubing of appropriate inside diameter that the hinge pin can work, stick it to the underside of the seat pan with JB Weld, epoxy or whatever, and go for it. Of course this assumes you will either use the original latch or replace it with a homespun alternative, all of which is 'do-able' with a little ingenuity. If you have a bolt protruding from the bottom of the seat pan on the other side from the hinge, so it lands just inside a frame tube, you could use a wrap of 'plumbers tape' on the frame, drilled for the bolt, lock washer & wingnut, you're done. Plumbers tape is that strapping you get for hanging pipes from a ceiling, it's drilled every 1/2 inch or so with varying sizes of holes that can be drilled out if need be. Or you could make a strap out of light aluminum, drilled at each end, wrap it around the frame tube.
If the seat is only attached on the hinge and corresponding lock point on the other side, and the seat pan is otherwise flat, it may tend to move a bit. You can stick a few blocks of scrap plastic onto the seat pan tight to the inside face outer frame tubes, with JB weld or epoxy. They will work as stops so the seat can't wander around.
That's what I did with a few 'TT' style seats I made from fibreglass as a kid and it worked well even on my motocross bike that got pounded all day and dropped every now and then.
