I think your concerns are well founded with regard to the loading of the plastic style log car. The stakes on the metal style cars do help to contain the logs when dumped from the ramp. If you have ever seen one of the metal cars up close, one side has fixed stakes, while the other side has stakes which drop down to allow the logs to roll off.
The plastic style car may not even unload into the bin of the ramp properly. Instead the logs may fall down into the gap between the car and the ramp bin.
Used postwar metal log dump cars are not uncommon, and usually aren't that expensive. $30 - $50 maybe. I haven't priced them recently. Check ebay.
Here's a pretty good looking one for a decent price.
Here's one with the original box.