with a TL, the design is only half the battle...testing the **** thing is what takes the time and effort.
Basically, a TQWT (tuned quarter-wave tube) is a design that completely eliminates the rear wave of the sub, tapering from around 2.5*Sd down to Sd at the opening. Stuffed with polyfill to slow the wave down through the length of the line.
Do a Google search of them, there are some excellent websites out there that give some theory and building tips for 'em.