Yes, definitely some type of firm cushion.

Firmer than softer - it needs to provide a firm rest without moving about or "settling" while trying to line up the shot.

Anything is better than the edge or top of the milk crate !!!

Even a sack filled with a lot of sand is fine.

Another thing. Strap or tie the milk crates down
so that they don't move - or fill a few sand bags with
sand or dirt at the range and use them to weigh them down.

You don't want them moving but on the other hand, the reason I use milk crates is they are cheap, easy and you aren't lugging heaps of weight around (as you can leave the sand bags at the range !).