You'll laugh but polypropylene shopping bags with 70%perlite30%soil, volcanic rock dust, worm castings.
The bags they sell you for about a dollar to use instead of plastic bags well they are made of polypropylene which is what smartpots are made of, I use them or I use if going smaller 10 litre buckets drilled drainage done by myself, bucket in bucket, but believe me the bags are way better and in the 70 /30 perlite and soil mix I can not overwater, in fact you can have the bags sitting in water e.g. if you need to go away for a week yes a whole week, even further the saucer you sit the bag in can be provided with an airstone it's failproof stuff no losing plants