Such as creating false bottoms in coke, water bottles or detergent bottles. A good way is to get a water bottle and to cut it off at the middle of the logo and melt a square of plastic onto the base so you can fill the top of the bottle. Then place the top of the bottle back on to the bottle.
If the bottle is dark colored and not clear you can just place items in the base but if clear such as most water bottles you will need to create a second bottle where the logo ends.
Bad locations are in your shoes, jacket, pockets, though false pockets are good. Common items with false bottoms are a great place to start for making a travel stash.
If you can’t be bothered to make your own bottle then you can just buy one from one of the following websites.