All you need for this great GTD system is letter sized paper, a binder clip and a pocket knife.
I went through several iterations of stiff plastic backings but eventually realized a folded 8.5x11 sheet can give enough stiffness to write on. The folded paper backing is nicely flexible for when you don't have a shirt pocket and can also be printed with reference information if you like.
I sometimes use a template to print my contact information on the back side of pda sheets and use them as "business cards."
PS: I use a mechanical pencil (0.5mm Pentel P205) when I have a shirt pocket and a Zebra F301 Compact pen (5 bucks for 2) when I just have pants pockets. I would love to use the Fischer Space Pen but it costs 24 bucks and I lose pens occasionally.
Step 1: Cutting Paper
Fold a letter sized sheet in half. Place the folded edge at the edge of a stable flat surface (such as your desk). Cut along the fold keeping the knife plane flush with the surface of the table. Be careful! Always position your joints and body such that if you slip you will not be cutting yourself.
It is helpful to use a slight slicing motion while cutting rather than just pulling the knife through the fold. The slight slicing motion makes the edges smoother but it is also a little harder to keep the cut exactly in the center of the crease.
Concentrate! This is a little like doing origami.
Step 2: Second and Third Folds
Repeat with the 4 sheets to create 8 sheets. The final cut is going through 4 thicknesses of paper at the fold. It is important to fold carefully and cut carefully.
Step 3: Backing
Fold one sheet of paper in eights to make the backing of the PDA. If you have a piece of stronger paper, great. If not, a regular sheet will do fine.
Again, I have tried cutting a piece of plastic (from a plastic clipboard) and sanding it smooth on the edges. This works well but I find I am wearing less shirts with pockets these days and I don't like the stiffness and bulkiness in my back pocket.
Also, I inevitably misplaced the PDA at times and felt stuck without the backing. Now I can make a new one almost anywhere in a couple minutes and be good until I find my misplaced notes. The paper backing works better. Isn't paper cool?!!
If you want to get fancy you can print important reference information on this sheet and just unfold to refer to it. When you are ready for another backing just print another one.
I keep emergency contacts (personal and work),credit card cancellation numbers and IP addresses on this sheet.
Step 4: Assembly
Step 5: Business Card
I like to write something on the page to remind the recipient about our interaction or what they said they will do for me.
I uploaded a template I made in Microsoft Publisher. I could not get the spacing right in Word. Maybe you have a better idea for the template and can email me?