You'll been able to find some info on this site about home brews for tire preps. I've not used any myself to be able to speak to how well they worked. I know a handful of people who make their own prep. Some of them are successful racers and claim to make a gallon for $30-$40. Look at the dozens of prep manufactures out there. They thought they could make something as good as or better than what was already available. I agree with the other comments that it comes with risk, both health and cost wise, and without support. However, I wouldn't let people talk you out of it if that's what you want to do.
I'd suggest reading the sticky in General Karting Forum "A go to thread for the new or newer dirt oval racer". There's some good info in there about making sure your driver is consistent, preparing and maintaining your kart, and where to look for improvements. I'd also talk with other racers at the track and see what advice they have to offer. Maybe they watch your driver or take a look at your kart and can provide some input.