Firing Range Barricades...

On youtube, look up IDPA & USPSA matches. Lots of barricades there.

Avoid overbuilding. Barricades that are heavy & unwieldly will end up not being used as too much trouble. 2x2 and 1/2" plywood construction (use construction adhesive and screws to assemble), with triangular ply gussets at the stress points, and you are good to go.

Consider that their 'legs' need to provide stability. How wide is up to you - I'd make some of varying widths to force movement from one side to the other. It is nice if the barricades can somewhat stack together in their storage locker.

Use what you would on the job. Junk cars are good, as long as they still roll. Shooting over a car is not the same as 2x4 stuck in the ground like at a PPC match.

Keep it simple! Use some wood with points cut at the end so you can stick it into the ground (think stake, but not cut short) and use some of that orange plastic construction fencing material. Weave a few loops of it through a stake, cut out holes where you want "windows" and put another stake at the other end. Sure, you can see through it, but if it's training you only need to simulate a barricade.

large plastic drums work well stacked up too.

Consider building a VTAC barrier.

There are some great videos on YouTube of how to employ these barriers in training and competition.

I believe you can find the plans to build them in the book "Green Eyes and Black Rifles."