Saw this design recently: http://www.redmaloo.com/
but two things immediately struck me, no handle and no where to store a power supply or mouse. So....
Note: as of 9/12 The bag is complete but havent added pockets for psu, mouse & bateries
Step 1: Materials
Materials are irrelevent. Although the felt case from the Maloo site may cause some cooling issues for powerbooks. I wanted a moderately durable material and opted for this 1/8" thick antistatic workbench rubber mat. Some neoprene for the interior to provide some shock absorbsion and a combination velcro & tarp snap for fastning.
Step 2: Plan layout
While I like the layout from the original idea there is a slight issue, most spare material I have is a yard wide and a 15" laptop wont be easily covered. I've opted to have a long fold across the width of the powerbook. Fastners will come in from the top and bottom, and the long flap will fasten with a handle.
In the end this design 'limitation' allowed me to include a mouse mat area. For the accessory storage requirement I'm going to attach a stretchy material to the long flap (what is the underside in this image). So, the long flap can go both under the short flaps for a tidy looking case, but along the outside of the snap/velcro short flaps if you're carrying extras. I'm going to head over to tj-maxx and pick up a nylon sports shirt with a nice pattern and adhere it to the outside.
Step 3: Cut the neoprene
I cut the neoprene so that when the bag is folded together it forms the interior of the bag. One of the final steps is to adhere it to the rubber mat with contact cement. Added some fabric spacers to allow a little air under the powerbook, which can get bloomin hot let me tell ya!
Step 4: Add the snap fastners
These are heavy duty fasters for tarps.
Step 5: Handle
I'm sure there's a proper name for this type of arrangement, if someone would like to post a comment I'll update this step
Step 6: Finishing touches
velcro between the snaps
rounded the corners