OK, I took the MLTL sim and turned it into a ~63.38" long x ~153"^2 open bottom TL, raising tuning into the 40s with a ~40% driver offset to minimize frequency response 'ripple' = 36.5" from the bottom, so several inches of elevation to experiment with, which normally is more than enough as you want the floor to preload it a bit.
The sim blue line is with the MLTL's minimal lining of the top, one side and back with the red line a 100% fill with ~1.4 kg of polyfil.
Stuffing just the top half or so won't be enough to damp the lower mids 'ripple' without heavily rolling off the [mid] bass plus the upper half from ~the driver centerline - top is half the stuffing density of the lower half to minimize mid-bass loss, i.e. this is the area where one normally should adjust a pipe's damping whereas a vented alignment is the opposite.
GM
