Cover for Vanguard Heater Return/Spa Drain

[ddb]

2006 HS Vanguard,  somehow the cover for the heater return/spa drain has come loose and won't screw back in or stay in.  I don't know if gluing it back in (after the next drain, before refill) is a possibility or even a good idea.  I can't seem find that part anywhere, dunno if a new one would screw in properly or not.  I welcome any thoughts about the best way to fix this.  Thank you.

[TwinCitiesHotSpring]

http://www.backyardplus.com/proddetail.php?prod=72723 if you want the white version its #72722 which can be found on the same site just type it into the search box...I know they have years listed but the drain has been 1.5" for years and I doubt will ever change so it will work on your Vanguard