Rather than Sonotube I ended up with a 5- gallon bucket. Very strong around the sides in expansion mode from air pressure, but top and bottom must be made quite strong. I use my GTI to haul big and heavy stuff on occasion so reinforcing the cargo panel was a side objective. Birch plywood panel, bucket height to match correct volume, puck on bottom with precise height so that about half the weight of the whole assembly is carried by the bucket, putting the bottom panel in constant tension, making it much more resistant to expanding than it otherwise would be (has to lift that weight to grow in volume appreciably). Black carpet is reinforced with heavy nylon mesh so it won't sag into bore holes. Felt furniture slider on puck to eliminate rattles. Guitar amp pocket handle on panel. Parts express driver. Response is nice and tight and the whole thing pops out in 20 seconds. I'm an old school SQ guy, not SPL, so this is a good solution for me.
In case others could benefit...