If you are looking for cheap boxes, I suggest the following.
If you have tons of weights, you can stack up 4-6 45lb or 35lb bumpers to get various height combinations. And when you have a taller partner, you can add a 10lb or 25lb in between switches it works well.
If you don't have tons of plates and no carpentry skills, buy a valve box as your base and then add pads or a bumper plate as needed to get the needed height. I've used the valve box for steps ups, low box jumps and box squats with around 370 on the bar and it has been rock solid. Not sure if the box could hand a heavyweight and 500lbs squats, but its been fine.
I guess if you are really short, a 12" box might be a tad high. For me at 5'6", 12" takes me to at least parallell.
Finally, if you don't have rubber mats lying around to cover the hard top, you can buy a cheap stadium seat cushion at a sporting goods store, or use a gardening kneel pad.
In addition, if you ever want to squat into a foam top, just head to a Joann Fabrics or other fabric/sewing store and for $25 you can get a 4" foam top.