"[In the West of the Temple]...is an upright coffin, or Tomb."
"...and so to the Tomb in the West.) She draws her Sword and pulls down the Veil therewith."
"...He goes out, the DEACON and Children following, into the tomb of the West."
"...the Tomb is stationed at Malkuth."
-Sabazius' notes on Liber XV
From a practical standpoint, the Tomb needs to be large enough to accomodate the Priest (and his Lance), the Deacon, and the Children. It needs to have a veil which can be opened by the Priestess' sword. Beyond that, it's pretty much open.
Common desirable qualities for all Mass furnishings are that they be portable, lightweight, sturdy, and washable. (Even 24/7 spaces need to be rearranged now and then.)
Most Tombs I have seen are black, though I've also seen red, blue, purple, and white veils. I've seen them made from PVC and copper tubing, fabric hung from the ceiling, a fabric "door" to a closet or doorway, and even fully fabricated from wood.
Now for what I'm doing:
The frame for the tomb consists of two garden arches, about 5' wide by 6.5' tall (they could be up to 7.5 feet tall, but since our prospective temple space has a low ceiling I opted to lower it. I haven't met any 7' tall priests yet, so I think this will be okay!). Each arch alone is only about 15" deep, so I'm placing them back to back to give the entire thing a depth suitable for accommodating 4/5 of the Mass team without bulging out the front veil.
The arches are similar to the one shown here in basic design: http://ourworld.compuserve.com/homepages/FLAIRMET/RAStdd.gif
These arches have the advantage of being very lightweight and portable. Completely breaking them down would take some time, but when done the arches take up the space of a couple of textbooks and weigh far less. For most cases I anticipate it shouldn't be necessary to break them down into more than a couple of pieces, which would take mere minutes and only require a screwdriver.
The cover will be mostly black; cotton, for its light weight and washability. There will potentially be a weighted trim along the inside of the bottom, should the frame require more weight to hold it solidly in place. The black cotton should be opaque enough since there will be no light sources within the tomb. On the front, there will be an inner and an outer veil. The outer veil is primarily for show, and will be tied back with curtain ties or something similar. It will be the same black as the rest of the cover, but trimmed on the left (as you're looking at it from the center of the temple) with olive and on the right with russet. The inner veil will be citrine. This gives the entire tomb the color scheme of Malkuth, but relatively subtly (and I think elegantly).
The inner veil will be in two panels, the top of each slanting down from the center to the side on its own rod. The panels will be fastened together in the center with a large-ish hook and loop, or some other fastening that is easy to undo with the point of a sword, at which point the inner veil should simply fall open.
I have the arches ($8.88 each from a local crafts store) and the fabric (10-15 yards of black cotton, 5 yards citrine silky something-or-other, 4 yards of olive and russet 1" ribbon, total cost ~$60). I'm currently trying to figure out how to sew the back panel to the top in a smooth curve.
I still need to acquire the rods for the front veil and figure out how to attach them. This will likely be two dowels connected by a hinge.
As for pictures, right now this is all you get: