okay a question just for me
my wife does a mail route and all mail people rubber band your mail before they put it in the box.
My wife uses # 19
rubber band yes they have numbers those are nice they strech really long but you have to use 2 at a time over a deep for the comb to stay in but they strech the whole 19 inchs over a frame long ways you never see anyone do that but it keeps really staight comb. http://www.amazon.com/Alliance-Sterling-Rubber-Band-Inches/dp/B008X09SZM/ref=sr_1_10?s=office-products&ie=UTF8&qid=1373997730&sr=1-10&keywords=size+19+rubber+bands
Right now I am using # 33 rubber bands
they are strong enough to use only one over the 9 inch length on a deep frame but still strech enough to use over the 19 inch full length so I am using 33 right now and if there was something between 19 and 33 I would try it but that was the options at the office depot http://www.amazon.com/Universal-Rubber-Bands-Size-Pack/dp/B0017D7L9C/ref=sr_1_3?ie=UTF8&qid=1373997682&sr=8-3&keywords=size+33+rubber+bands
using medium frames and you might find a completely different size to use