water heater drain cap

if its a metal valve screw another garden hose valve to the first.

or regular caps are sold at home depot for a few bucks.

Go to your local hardware store and get a cap that fits a 5\\8" garden hose. I've done it more than once. This is probably a good argument for NOT periodically draining water heaters. I've drained some waters heaters I've owned and not others and the only consistent results I've gotten were leaky valves.

If it HAD been drained regularly, then the valve would not have gotten crap in it and began leaking.

I'm not sure I believe regular draining prevents valve leaks, either. I just replaced the leaking (plastic) valve in my 6 yo gas water heater. I have drained only occasionally, and the water always came out clear. The old valve had no "crap", and was totally clean. I believe the washer had deteriorated and probably could be replaced to repair the valve.

I bought a quality hose bib (the ball valve kind) to replace the old valve. I thought I would have to totally drain the heater, but I didn't. I closed the inlet valve and put a bucket under the drain valve. I turned to pilot setting and waited a few hours for it to cool somewhat. I unscrewed the valve and although some water dripped into the bucket, it didn't all come out. I worked quickly to screw in the new valve with teflon tape. I doubt if over a gallon came out altogether. I suspect a vacuum forms that keeps the water from draining, as long as no taps are open.

I was too dumb to think of the cap trick, which would be less work. Probably cheaper too.