If you're using steel shims I have an idea. Get some steel tubing big enough that would fit around the tubing your stand is made of. Split it to make channel steel. You might actually be able to get channel that would work, so you don't have to split it. Use that toake your shims. If you need more space just tack some plate steel to it to space properly. You'll have to notch around the cross bars that run front to back. That way there would be no concern about any of your shims moving out of place, since they would be fitted around the tubing on your stand.
