I know you want to fix it quickly and cheaply, but you should also do it properly and more permanently to avoid future leaks, so I would not use a rubber coupling. It may also not be to code to use that type of rubber coupling for this application. Measure the outside diameter of your drain arm and your galvanized pipe. Then take that picture to any big box store and get the following parts in the correct sizes: -a threaded, galvanized female coupling to go onto the end of the pipe coming out of the wall -a PVC fitting that screws into the new coupling -an adapter fitting, like the one in the link below -a small can of PVC glue -a roll of teflon tape This should cost about $10-12 total. Temporarily disconnect the P-trap arm. Clean up the threads on the pipe with a wire brush, wrap the threads in at least half a dozen turns of teflon tape, screw on the galvanized coupling, and tighten it down with a pipe wrench. Then screw the threaded PVC fitting tightly into this coupling. After that, glue the PVC adapter to this fitting; you might need a short length of PVC pipe to connect the two pieces. Reattach the old P-trap arm: slide it into the new adapter fitting and tighten down the nuts at each end. If the old P-trap arm doesn't reach far enough, buy a new one, which will be longer. This sounds more complicated than it is. Once you have the parts and see how they go together, it will be easier. Or, you can spend $200 for a plumber to do the same thing. Finally, don't avoid using the galvanized coupling: if you screw a female PVC fitting onto the male pipe threads, the fitting could crack or leak. It won't leak if you screw a male PVC fitting into a female pipe thread as I described above. Here is a link that might be useful: Adapter

Allen wrenches, also known as hex keys, come in folding sets of multiple sizes that cost about $10-15; you are sure to find the size you need in the set. The folding sets also come in metric size in case your faucet was made in Europe or a country that uses the metric system. Just go to the hardware store and you’ll see both. Here is a pic of one, but all the hardware manufacturers make them.

Why limiiting the choice to Moen?

Thank you all for your suggestions. A Mat, I don't have this tool. To be honest, I have never heard of it. I tried tightening up the flexible connector to the faucet with some of the tools that I have for my vehicle, but it turns out that the size of the nut is too big for all of them. Normal pliers didn't help either. By the way, this is picture from under the faucet.