The drain cock someone linked to above is the kind I tried 2nd, I did not like it at all, it lasted one use and broke.. The one I got was very poorly built.
There are two kinds (Wel lmore than 2) of plastic fittings, On Attwood heaters you should only use plastic in that hole (Though I do use well taped brass myself)
Make sure you get the one rated for hooking directly to hot water heaters,, You need a 1/2 inch threaded adapter, and either elbows or straight pipe or a combination and a 1/2 inch BALL type valve (Quarter turn valve) you may need a 2nd threaded adapter to fit the valve (or not depending on the valve) and a 1/2 inch plug, your choice of metal or plastic for the valve outlet,, and a 1/2 inch nipple 4-6 inches.
High temp plastic to tank, and to valve tank siide Position valve so you can operate it, I had to trim the handle some on mine.
Plug valve outlet (Safety plug not really needed but I feel better with it.
Use a pair of zip-ties to attach the nipple to somethign inside the compartment.
To drain, Remove plug, replace with pipe nipple, open valve.
When I built mine I used the wrong kind of plastic and as a result had the devil's time cleaning it out (A 1/2 inch pipe tap would have made it easier) I replaced it with a well taped brass drain cock,, It lasted one use, so I got a better one at Camping World.. So far it's still lasting but it's only been one summer (Installed last spring). I much preferred the plastic version.
Camping world link is what I have installed now,, Well taped with multiple layers of Teflon before insertion.