Since you have been waiting for an answer, I thought that I would suggest a solution that I have seen used. These are usually used in the classroom in conjunction with manipulatives. The 3-D look of the flats, rods, and ones should be changed to a face on view. However, as you have found, that presents a problem for the cube. These already take several pages to show, so I suggest using a cube shape for the thousands block. Show the smaller squares only in the front of the block. You could add a TN if you thought it necessary (depending on how they have been introduced, they may have described each shape in the textbook along with the diagram) saying that the 3-dimentional object represents the thousands cube. I'll also attach an example of what I mean. The Guidelines and Standards for Tactile Graphics does give preferred sizes for blocks of ten, sometimes called hundreds blocks). Top, side, and front views of these are really not necessary, and would just be confusing since they would look the same as the hundreds flats. Hope this helps!
TG Committee Member