Go to the home store, get a 5 gallon bucket, a large supply of heavy duty plastic trash bags, and a small round toilet seat, along with the kitty litter.
Line the bucket with the trash bags, put kitty litter inside (an inch or so at a time), put toilet seat on top. Use as normal.
After use, add a layer of kitty litter.
When bucket is approximately half full, seal bag, remove from bucket, place in another bag and seal, store in out of the way location. A balcony is ideal. Re-line bucket, etc.