At last, here's the final product.
I'll provide my feedback about setting it up in a separate thread.
For now, here's some pictures.
The finished turned better than I expected. With the 20 layers of MDF, I thought it would impossible to get a smooth finish, but overall it is not that bad. Three coats of water base enamel plus three coats of clear varnish (water base). That's it.





I added some feet. I cut four pieces of 2.5 inches of a 7/16" threaded rod. I threaded the MDF directly. Seems like strong enough to hold the weight of the sub (which turns out to be 96 pounds). I designed and 3D printed some cap to hide the screw (thread).



That's it for now.
I have been listening to it for few hours now. The integration with the X-LS Encore was fairly easy. I'll describe how I did it in a separate thread (it is more about setup then construction).
My next construction project will be to build descent stands of the X-LS Encore. Stay tuned.