Go into any bike shop and get a Specialized "Large Universal Slap Jacket" about $10-$15 at Supergo, Mike's etc., it's something to do with a mountain bike, but looks just the right size for a frame top tube, and they make the best-ever strap pad for a timbuk2 bag too :-)
OK just checked, perfect fit on the top tube on my Langster
freewheel on hayes.... they have like 3 different options. handmade ramblin worker, reload and some others maybe. choice of colors even!
have steve at ramblin worker make you a custom one, or buy one from freewheel ramblinworker.com he will make it the total lenth of your top tube so it will not slide around, he can also embroider cool little graphics. i have one with the hope diamond on it...
i have one from a lizardskin bmx set i bought to get a stem pad for my dj bike. it's black nylon with the lizardskin logo in white, but you can reverse the cover and have plain black. i don't need or want it, so pm me your address and i'll happily mail it to you