If the place has no ceiling, bring your own.

• White cardboard is an obvious cheap one.
• Styrofoam is lightweight and holds its shape better than cardboard (depending on the thickness of your cardboard.
• Any "professional" reflector will make you look more professional, but at a cost. For some clients it could be bad to show up with some cardboard. "Are you charging us for using the cardboard you took from our garbage?"

To hold it in place, either use a stand or yourself. This largely depends how you want to bounce the light. The cone of light from your flash increases in diameter, which means you need a larger reflect the higher you want to hold it.

Speaking of which: to determine the size of the reflector, take a few test shots at home in your usual setup but photograph a mirror. This way you can see the area on the ceiling that the flash bounces off. Depending on the zoom setting and the ceiling height you will find the area to be rather large (heck, that's the whole point of it and what makes the images look so good) You will most likely not be able to get all of this with your smaller reflector.

The result will not be the same as using the ceiling, but depending on the reflector you can probably still achieve good results with it.

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Prepare as much as possible at home, but most importantly: communicate the situation with your client. They are not photographers. They do not know that a white ceiling is important for your work. They want nice looking images and I see no reason why they would not want to help you. Maybe they have some equipment for their showrooms to hold up the reflector.

Depending on the time schedule and deadline, offer to take a limited set of images for a smaller price beforehand. this allows you to make yourself familiar with the situation and they don't have to go all-in on somebody who has not too much experience with this. If they like the images, do the rest. This also gives you and them time to think about the images of the first session and do things differently in the second session. It's not like you have one huge chunk of images and then they are like "...oh, you know what would be cool..."