I second using a a zip tie! I would pair it with a rubber band (not hair tie. rubber bands won't slip on the hair). You could also use a rat or pin a couple of braids in front into piles and then lay the rest of your braids from the front over that to get a poof. Are your braids marley hair that was braided into your hair? Or were they pre braided and you crocheted them in to corn rows?
If you want the really big teased out ponytail look you could install a drawstring into a half wig, bundle up all of your braids into a bun, cover that with a hair net or a pair of old tights, and then put the half wig of braids over that. The bun of your regular braids will hold up the half wig and make it bigger/give you more volume. Plus, if you used pre braided hair you could really easily make a half wig out of a dome cap with the braids you used on your head so it would match your hair exactly, and it would be pretty cheap to do!
If any of that sounds kind of like what you are looking for I can go into more detail, but I don't want to write a novel if its not what you're looking for ;) Just make sure when you pull all the hair back and pin everything in that the tension is distributed to multiple braids. If only a few braids are being pulled tight it can lead to premature hairloss (tension alopecia).