I built my own zapper by following videos on youtube. I used a breadboard (a circuit board that does not require soldering). The bad thing about that is that if you drop or bump it, wire can come out and then you have to figure out how to put it back together. I tried making another, using a soldering iron, but it didn't work. I think my soldering skills are lacking.
Also, the zapper I made does not have variable frequencies, which I think is something you'd want eventually, though not necessarily right now. There is probably something on youtube about building one like that. Oh, and the copper tubes you can get in the plumbing department at any hardware store. Most will cut it to the length you need. Mine are 6" long, 3/4" copper pipe.
For your cats:
I'm not familiar with Humaworm, but I see on their site that they have capsules for humans but they also have a product called Herbal Pet Wormer, which appears to be a powder. Are you giving your cat the capsules or are you using the powder? Anyway, this method works well for giving any type of pill to dogs or cats. Have the pill in hand. With the other hand, open the animal's mouth. Place the pill on the back of the tongue. Close the animal's mouth and hold it closed with one hand. With the other hand, massage the animal's throat. It helps to speak softly and gently to the animal while you're doing this.
If you're giving the powder and they won't eat whatever you've sprinkled it on, I would suggest trying something they can't resist even if it tastes a bit odd. I am thinking that the powder suspended in cod liver oil would be a good choice. If that still doesn't work, I would ask the folks at Humaworm about the possibility of subcutaneous injection for the cats. It seems to me that would work, too, but I am not a vet and I would urge you to get more information before proceeding with *that* advice!