If the requirements are not high, you can use a thick rubber pad and manually level the aluminum plate with a sledgehammer; Or leveling with a jack (this method requires welding a shelf with channel steel). The best way is to use a forging hammer, but it will increase the transportation and processing costs. In this case, you might as well use the pre-stretched board directly, and of course you don't have to worry about flatness!

The above leveling methods are all seen by my customers, hoping to help you.