As far as what to do to obtain an airtight cabinet, it's really not that difficult. Good build quality is all that's required. Don't line the seams with silicon, grout or gasket sealer. Just make your cuts true, form your joints well and use a liberal amount of glue.

After you've assembled the speaker and installed the woofer, tweeter and connector panel, give it some serious volume. You've run that bike with open headers a time or two I suspect, so don't spare the ears on this task, just for a little while. Now feel around the circumference of the woofer and tweeter and check for gusts of air. You'll notice it most on deep bass notes, drum impacts and the like. If your woofer and tweeter holes are cut so that there is a large "lip" of contact surface area, then you probably won't need any gasket material. But if you feel any little burst of air, then form a gasket from silicon or something - just a thin bead - and run it along the sealing surface of the offending component.

For more cabinet construction tips, check thorugh the archives by using the search function. Try specific words like "bracing" or "joints" or "insulation." There are some really talented cabinetmakers that frequent this forum and they have given some excellent tips.