It's really not very hard to make your own jig for very little bucks.
1. Buy a wood square dowel 1/4" - they are usually quite square and accurate. Available at Ace, HD, Lowes. A buck or two for 36"
2. Cut two, 4" pieces of the square dowel.
3. Laminate two 1x4 by about 18" and attach this jig to your miter fence to allow moving sideways - easy with the BT3 fence with slot. Also easy with the Rockler fence clamps if you have them (get some if you don't have...

Porter Cable 4212 (Dovetail/Box Joint Jig) - $146.96 - Amazon - Free Shipping

I've been monitoring the price of this model and the model...