I simply put 2 or 3 on a size 14 hook (I mainly use Drennan super specialist barbel hooks), or sometimes put a bunch of them on a hair rig, also Korda maggot clips are good for presenting a big bunch of casters.
When hair rigging casters get yourself a very fine bait needle otherwise you'll keep bursting them & the same applies if your planning on using the Korda maggot clips use the smallest ones, as they offer the better presentation.
I have had all my best Barbel on boileys [Spicy chickhen,Fresh Prawn and halibet] Pellets i find tend to pick off the smaller ones[under 6lbs]All my doubles have fallen to Boileys
I have been fishing the teme over the last 3 seasons & a bait that still works is a chunk of luncheon meat about 31/2" X 31/2" big bait for a big mouth biggest so far 11lbs. 8ozs.