I started by calculating the volume of my old speakers. I got 1.4 cubic feet for each speaker. I like the sound of the old speakers, so it seems the best plan is to use the same volume for the 2x10.
Next, I picked the width and height of the cabinet for a convenient size, and used the volume of 2.8 to get the depth of the cabinet.
I plan to use the same construction technique for the new cabinet: 3/4 plywood case with 1/4 plywood glued on as a veneer, then dress up the edges with 1/4 round. Matti can then stain it, paint it and/or polyurethane it.
The parts easily fit on a 4x8' sheet of plywood. I hope to get the store to make the major lengthwise cuts since I don't have a table saw, and it's hard to make those cuts even if I did. Otherwise I'll lay out the parts across the 4' dimension and use a straightedge and a circular saw.