Looking for DIY Atlas pack trainer ideas

I've always used sand bags in the pack I'm going to be hunting with but you could probably take a cheap sled modify it and strap it to a pack if you wanted to use steel plates for weight.
What current pack do you have? I've used sandbags, concrete, etc. and all work well.

If you have a pack with a load shelf make sure you put the weight there instead of in just the bag. For my SG bag the load shelf carries the weight significantly better.