Take a form-holder, add a custom t-square and rules along edges -> portable drafting tool with storage for drawings, pens, etc.

Project is pretty simple, though time consuming; total construction time was around 4-5 hours. Most of this is making all the rule marks, so if you sacrifice function a bit, you could get construction down to around 2-3 hours.
For materials, you'll need a bit of acrylic or polycarbonate (plexiglass or Lexan), a form holder (try Staples, about 17$), epoxy, crayon, and paper.
For tools, you'll need clamps, a file, a box-cutter, a saw (like a band saw) for cutting the plastic, sandpaper, and a square.

Step 1: Fabricate the T-Square

Cut the plastic into the shapes shown below. Some notes:

The 3.5" dimension is arbitrary. I feel that it is sufficient, but if you'd like, you can make it longer, to make the T-square more stable.
The 7/8" width of the small piece is also arbitrary, but works well. It is wide enough to prevent it from wondering too much during glue-up and narrow enough to not be huge.
Leave a bit of margin for error on each side of the straight-edge part of the T-square. This is because you will be truing it up so that it is perfectly straight and of uniform width, and you'll need some material to work with.

Once that's done, true up one edge of the straight-edge on the T-square so that it is straight. I tried a couple of techniques to do this, but the only one which I could get to work was optically:

With your eye, look along the length of the edge.
Mark the high spot(s) with a marker.
Clamp the piece to the table and file off some of the high spot(s). Make sure you keep the file square with the T-square.
Repeat steps 1-3 until you can't determine where the high-spots are.

File the other edge of the straightedge until the width of the strip was almost 1" - I went to about 1.015", on average - along the whole length, and then straighten that using the optical technique given above.

Similarly, straighten one edge of the small piece. (actually, I was able to use a factory edge of the Plexiglas, so I didn't have to)

I glued up the T-square using epoxy and the jig shown below. Make sure that the edge of the straightedge and the small piece are in contact with the square.

Make sure you clean off the square (using isopropanol [rubbing alcohol]) once the epoxy gets firm.

Step 2: Scratch the Rule Lines

To make the rule marks on the form holder, proceed as follows:

Lay down a strip of layout ink - or sharpie - along the edges to be marked. (This isn't strictly necessary, but is nice to do)
Protect the ends of the form holder with a bit of masking tape.
Set a pair of calipers to each measurement, put the top jaw against one of those masking tape pads, and use the bottom jaw to lightly scratch the aluminum of the form-holder.
I made marks every 0.2". Feel free to use fractional (rather than pseudodecimal) or metric markings.
If you are feeling lazy, you can make one just every 1", and the make five marks on the T-square. The measurement would then be "...7 inches, plus... 3 fifths of an inch, for 7.6 inches".
Once all of the marks have been made, protect the form holder with a piece of masking tape. I used one 3/4" from the edge, though this is probably too wide. You might prefer to go with something closer to 0.375-0.5".
Using a square to guide you, deepen the scratches. I used a box-cutter with the blade geometry shown below.
Make some sort of secondary marking to indicate the integer inches (1", 2", etc.) I chose to make a triplet of lines, extending out to 3/8" from the edge, as well as numbering them.
Remove the layout dye with isopropanol (rubbing alcohol). The 91% works much better than the 70%.
Darken the scratched lines by rubbing crayon into them.
Normally, working with fairly soft plastics (PVC), I would apply the crayon directly to the scratch, then rub away the excess, but with the shallowness of these scratches, I found that it worked better to apply the crayon to a piece of paper first, then use that ti rub it into the scratches.

The marking process for the T-square is similar, except that:

I made a Vernier scale along the short side of the T-square by making 6 marks per inch, rather than 5. Thus, it becomes easy to measure even 1/25th of an inch.
It is sufficient to simply mark the plastic with the calipers; you don't need to mark it first, then deepen the scratches.

To finish, garnish with some masking tape (the blue painters' tape is good stuff) and a small 30/60/90 square.

Step 3: Use

Use is pretty straight forwards: the glued-on bit of the T-square rides along the edge of the form-holder, allowing you to make parallel lines of known length at known locations on the piece of paper you tape down to the working surface. The 30/60/90 triangle allows a second way to make perpendicular lines, as well as 30 and 60 degree lines for perspective drawing, hex nuts, etc.

A couple small rubber bands help keep the pieces together inside the kit.

And just for fun, the carefully selected tools inside the kit also include: mechanical pencils, 0.7mm, 0.5mm, 0.3mm; permanent markers in black and purple; box cutter (Olfa autolock); highlighter; paperclips (Regal design, not Jewel); Zebra Z-grip medium pen (for drawing, mostly); Atlantis pen (for writing words, mostly); eraser stick.