Usual trick I would think of is to make a simple veggie broth and freeze it after reduction. Tomatoes and cauliflower stems should be good for that and watermelon same but juicing it and freezing it.
At least it stores better and longer and reducing the air and space it takes up reduces it as well once it’s just a liquid. Freezing in baking pans helps it go quicker to even though it is more smaller batches.