The cardboard tube is a good way to join it together, but I use PVC tube on mine since you can attach a cap to one end which makes it easy to attach a stud since you can tap directly into the PVC.
However, I think your soft parts selection is bad. You do not set up the soft parts for a PR
like you do a sub. You want the softest surround and spider you can find that will support the excursion. Multiple spiders and stiff surrounds lower the efficiency of the PR
, and it will not come close to the output of a port. Also a PR
with a stiff suspension will have a high fs which is counter productive, unless you add a butt load of weight which aids in lowering the efficiency.
Don't post links to other websites in our forum please.