Well, quite a construction, well done. But looking at it a second time, I wonder why you still use the tube, why not leave it and have a rigid wooden construction?
For the Peerless XLs10 the price in Dutch Guilders is 350,- (should be about 160 USD)
You can have a look at the complete design (speaker, radiator + power plate amp 200W) at the site if the German speaker company "Intertechnik"
The sub goes down to f3 18,5Hz (within linear range)