My favorite herbs for the kidneys are nettle leaf and schisandra berry. Both will help to strengthen the tubules. Powders or capsules are the best since the fibers will provide some silica that strengthens the tubules. Tinctures and other extracts will provide a lot less silica.  If you want to make a mixture of the two I recommend 3 ounces of nettle leaf powder and 1 ounce of schisandra berry powder blended together well. Recommended dose is 1/2 teaspoon 3 to 4 times daily at least 20 minutes before meals.

Adding food grade diatomaceous earth to your water will also help by providing silica to strengthen the tubules. There are a number of posts on this forum on doing this.