Why BMI Is a Crock, in Pictures. So, the Guess The Rotund's Height and Weight game gave me an idea. (Oh, and hey, if you want to see a scatter graph of the results, there's one here now.) I talk a lot about how BMI is bullshit, but we all know talk is cheap.
As suggested in the above links (Thanks! ShellaB, for posting those links -- The first one was especially useful.), you can use monofilament plastic fish line to tie the fern to the basket, if needed. I stuffed my staghorn's basket with a combination of leaf litter, spaghnum, and Spanish Moss.
In general, larger pots, perhaps gallon size, will allow the ferns to spread their roots and achieve maturity. Your type of fern prefers moist to a bit dry, humus-rich soil and shade. The pots you use should drain any excess water well. This will not only prevent soggy roots but also prevent salt buildup.