Abba_S,
Maybe you could attach something to the walls of the tub to make it big enough to hold 40 sa’ah of water, but the water is still coming in the from the regular faucet.
Now, there are those that hold that water that comes in from regular plumbing is not sh’uvim but if that is what you are relying on, then you can probably use the hotel pool too (as long as you confirm that they do not fill the pool with a hose).