Get a wide, extra-large size belt. (cloth is best)
Pull it around the seat at chest level with you in it and buckle/lock it, makeing sure the any metal part isn't actually going to grind into you.
Honestly, I had 0 problems with moving after this, even on our Kart track out here. Cheaper than anything else, and just as good, if you're not looking for crash protection at least.
Don't waste another minute waiting on parts, or anything else. Just get out there and start racing! You will suck at first, and you will be slower than many other cars, but the only thing you will regret is not starting earlier...