give ourselves a little extra gap for easier assembly
make note about threaded rod for long screws

long-ish screws can be found at http://www.mcmaster.com now, but may be
hard to find in the future ... but threaded rods are common
replace magic constant with descriptive name
add "compression gap" so sides are held more firmly together by screws
reinventing the wheel, 'cause that's the way we roll

removing sharp corners from the tab relief
round out slot relief and reduce tab length
add ventilation holes, but openscad will not finish rendering the 3D version
s/ventilation/vent/g
reduce number of vent holes to work around openscad crash