That second one looks pretty straightforward. It looks like half a log on top of a stool.

I would find a flat board, screw 3 or 4 half-circles on top of the board (perpendicular to it), and then screw little slats to the semicircles (parallel to the original board). Sort of like an old-style barrel. Put a couple bolts or eyelets into that, and use them to tie the saddle down tightly. It would be lighter and cheaper than half a log, but still pretty sturdy.

(Disclaimer: not an expert and I've never tried anything like this. This was just the first thing that came to mind.)