Yes the lower cradle was probably just a "halfpipe" with two grooves for the saddle rails.

This type of post is not very gentle on the rails, so probably not a good choice for saddles with hollow (tubular) rails, or carbon or aluminum.
Basically it's a bad design with one checkmark in the "pros" column, namely easy/cheap to make. The missing groovy halfpipe can be made with a vise, a hacksaw and a rat-tail file once you determine what diameter of pipe fits in the cradle shape at the top of your post. The top of the post could even be reshaped to fit a different diameter pipe if you can't get an exact fit.

Or if you have a place near you that sells used bikes/parts, they may have a bin full of loose seatpost parts. My LBS (Recycled Cycles in Seattle) has several bins of seatpost parts — they've never failed to get me the part I need. You might have to buy the whole seatpost to get the part, but ugly used posts with gouges go really cheap there. Scavenge the parts you need and throw the rest in the metal recycling bin.