Well I wouldn’t say it’s impossible. But I also can’t say it would be effective. First you’d have to build the riser with the proper dimensions to correctly facilitate the size needed. Also, I don’t know how it would work with room acoustics. I would recommend in room subwoofers, as I just have no knowledge about building a riser into a sub.
My choice would be in room subs with this...
The Hideaway Theater
r.tapatalk.com
It’s mostly for tactile response, and delivers couch shaking power below about 20hz, but you can set the slopes however you want. Some people do use it with the IR subs off as to not wake anybody else.
For building the riser into an actual subwoofer, find the driver you want to use, and then use bass box pro, or WINISD to model the driver for the correct dimensions. Could be cool...