You could try find tree pots to accommodate your current seedling long tap roots.
My experience in trying to grow maprang in So Cal is that they do not do well when the humidity level drops to 1 digit. The plant could never recover from 1 digit humidity level and would slowly perish.
Currently, I'm germinating some seeds and the plan is to try grafting the seedlings onto mango rootstocks. I've done this before way back in 2012 or something. It seemed to work as the scions flushed 3-4 times before the whole plants died. I'm not sure if that was due to incompatibility or care issues on my part. So now that I have more times, I'm going to try this again. But it will be my last attempt at growing maprang in So Cal.