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 them)
4. Use the dado blade set to precisely 1/4" wide (check by using the dowel pieces you cut - should be a slip fit) and cut a 1/4" high dado across the bottom of the attached wood jig fence (don't cut your BT3 miter fence!).
5. remove the attached jig fence and place one of the 4" dowels into the slot you cut, so that the majority sticks out in front of the fence. Fasten in place with glue, brad, whatever.
6 replace the attached jig fence back to the miter fence with the attached dowel to the left of the blade - use the second dowel as a spacer or setup bar to the blade to set the embedded dowel 1/4" away from the blade. Secure the fence. Run the Miter fence forward and backwards, a second slot should be cut across the bottom of your jig fence, parallel to and 1/4" away from the 1st slot.

Now you are ready to cut. Place the workpiece end down from the right side up against the dowel. Hold firm to the jig fence and cut - forward and then back.

At this point you should check, you should have a 1/4 x 1/4" finger and a 1/4 x 1/4" slot. Check the fit using the dowel. SHould be a slip fit.

Now set the slot you cut over the dowel attached to the fence, hold the workpiece firmly to the jig fence and then make a forward and back cut. Repeat until you have a row of fingers. (its best to make a box wall an integer multiple of the finger width.)

So it's easy. Typically you can cut four or more ends at one time, really speeding up the work. This jig costs you 2 bucks or less, a bit more if you don't have some 1x4 stock cutoffs lying around.

Some additional notes:
1. The BT3 has a bit of weakness in that the SMT table may not be completely level with the main table. This is critical to making the fingrs the rigth depth. The BT3 can be adjusted by using some or all of the following: A) add or remove plastic sliding shims from under the SMT clamps where they ride on the rails. B) adjust the rails - they may have some play with the quick release knobs that hold them to the main saw body. C) make sure the SMT does not rock or tilt while cutting.
2. Fingers of 3/8" or 1/2" are equally easy to make if your dado blade and dowels are those dimensions. If you screw the Dowels in so they are removable then you can have multiple sets of cuts on the same jig to switch back and forth.
3. Instead of wood square dowels, Use metal square rods (also available at lowes, HD, ACE) for more durable guides and setup bars.
3. Typically you cut the depth of the fingers about .050" or a bit less than a 16th oversize and sand them off or use a flush trim router to smooth out the corners when the box is assembled.
4. when cutting multiple box ends, don't forget you have to alternate the top and bottom after you cut them... Think about it...! There's two ways to do it.