I have the same problem. I mix DE in my soil mixes which holds enough water to give me a day or two extra. That has helped a lot. Plus DE has so many benefits to plants. I use 1/2 part, but you can use as much as 30% of your mix. Mine is like 5-10%. I can’t afford 100 containers with 30% mix. It still helps a lot at 5%. I use a 30 gallon sometimes 20 gallon containers. You can get DE here. It must be the size of perlite, not the powder! This stuff!! Best price I found.
Here is a study on the benefits of DE, Hundreds of studies confirm.
https://www.researchgate.net/publication/260133195_Effect_of_diatomite_on_growth_of_strawberry
Here is the conclusion of the study
Results obtained from this study have shown that diatomite is an effective
amendment to improve water holding capacity of light textured soils. Among the
substrates tested, the highest water retention capacity was obtained from 30% DE-I
treatment. As DE-II application has also increased water retention capacity, it was not
much more effective as DE-I due to surface area. Use of diatomite as substrate not only
improved hydro-physical properties of soil but also vegetative parameters of strawberry.
As a conclusion it can be said that, use of diatomite as a growing media not only
improves water holding capacity but also minimize leaching.
More info from the study
Addition of diatomite to substrate increased development of cull strawberry seedlings
(Table 3). Root number and length and leaf area increased significantly with diatomite
addition. Primer root number is directly affected by soil moisture in the upper layer.
Increase in primer root number and length can be due to increase in water holding capacity
and pore size distribution of media. Leaf area is important for photosynthetic efficiency. It
not only affects plant development but also yield capacity. Increase of leaf area is especially
desired in plants with low canopy, such as fern cv. Diatomite application increased leaf
area. The highest results were gained from 30% DE-I treatment