At ECHO in Ft. Myers, FL (www.echonet.org), they have a rooftop gardening area, where many plants are grown on a concrete pad in various and sundry containers. Including carambola trees in trucktire planters. Fruit on trees about 4-5 feet.
Agreed that the grafted and named trees are better, but most of the seedling fruit pretty good as well.
Recommended course I heard was to plant out the seedlings, then graft the best flavored ones onto the roots of the ones less so.