I used to believe I could build one for personal use of a patented technology.
NOT true, do some searching on it you will find they could come after you.
Reality; unlikely they will come after you unless you win the lottery or rich.
You must have something they can get to make it worth while suing you.
Could be a cease and desist order, risk contempt of court not complying, Jail.

Two details are missing,
One where is it patented and where are you.
If different countries... patents must be patented in each country they want protection or get a world patent $ (very often not done). If you patent in USA without world patent someone else can copy and sell it except in USA.

Lastly go back to the patent, they must reference similar technology. The feature you mention is something I read about more than 20 years ago. They may have made it work, thus the patent, so if you make it work differently its not covered by their patent.

If you're still in love with the design, tell them your an experimenter and would like to validate their design... need their permission to copy it. If they are not in production with it they may say OK.
Keep copy of OK. (you will not be allowed to sell it most likely).