in terms of strength (hollow or not) the usual advise is:
- have a fan blow gently since seedling and thru out, to encourage sturdiness, and
- adding silica. the trick is getting a good soluble source. theres ArmorSI but i believe thats not the best soluble (ive had issues with it mixing with my feritilizer). maybe PowerSI?.. also you can ammend with rice hulls and wallastonite (but solubility/time is an issue)
additionally to Silica, you can add fresh coconut water that provides cytokines which help improve cell development.