For the really small flatcars like this one, I will either put small lead weights between the frames where they won't normally be seen, or put a load on the car with the weight inside the load. Most of the normal HO standard gauge cars like the Athearn or Roundhouse kits come with a steel weight so it's not too much of a problem there.
If you decide to get back into model railroading, check out the On30 model stuff. You can get into it without breaking the bank too much, and the main difference is that you can use HO track to run them on instead of having to buy expensive On3 track or hand lay the rails. You can also buy really nice On30 flex track with the proper tie size and spacing, and it costs less than On3 track, if you can even find On3 flex track...
I've been modelling in On30 for about 20 years now, since Bachmann came out with the first ready to run stuff. It's definitely easier to super detail the O scale stuff, especially with these old eyes... :-)
Check out this post I did a few days ago about one of my O scale narrow gauge projects.
@amberyooper/hobbies-on-steem-my-o-scale-narrow-gauge-ore-cars
RE: Model Railroad Kit Review, an HOn3 flatcar kit