This isn't the answer that you're looking for, but the best way to prevent secondary poisioning from rodenticides is to keep your cats indoors.
Keeping your cats indoors protects against MANY problems that can make your cat sick or die! Cats who are either part-time or full-time allowed to roam outdoors alone face the following risks:
- Disease (feline leukemia (FeLV), feline AIDS (FIV),FIP (feline infectious peritonitis), feline distemper (panleukopenia), upper respiratory infections (or URI))
- Parasites (fleas, ticks, ear mites, intestinal worms, ringworm (a fungal infection))
- Car strikes
- Animal cruelty (neighborhood children)
- Injury from other animals (both wild and domestic)
- Toxins and poisions (in addition to rodenticide, antifreeze is sweet and delicious, but deadly!)
- Trees (if a cat can't/won't climb down, he'll get dehydrated and weak, eventually falling)
Source: Indoor Cats vs Outdoor Cats
If you're determined that your cat should be allowed outside, the best solution is to build a protected enclosure (often referred to as a "catio"). There are a ton of resources online for how these can be built/bought, performing a google search for "indoor enclosure cat" and "catio" turns up a lot of resources.