10cm x 10cm will be nice enough. laminate. not too big not too small.. can use velcro and apply to the flash head and the card itself . zek has done one b4...he gave me one sample but i think i somehow lost it :bsmilie:
I now use cardboard or 3 pieces of A4 paper folded into 2 and then folded into a shape resembling a triangle and then use 2 rubber bands. Bend it a bit and voila - I have one in 30 seconds even if I forget - and I can choose very white paper or slightly yellowish one to warm up the flash.
I used to use Lumiquest softbox but you know what - after a few years the plastic yellows with age and then you get warm colour balance all the time.
For 550EX, 1 x Gutter (yes, the army type) would be just nice. The hooks make it easy to attach/detach.
But for 420EX, gutter may be a bit loose. Can try going 2 rounds?