A lot of stress?
A fraction of the bike weight laying static on a flat surface?
I have seen or read people using plywood, tiles, any any sort of other material with success: what makes you think this is a stressed component?
Suzuki didn't put much engineering in the foot pad, otherwise they wouldn't have made it with a hollow base.
The only thing to be verified, once the extended foot pad is installed, is that with folded kickstand it doesn't rub against or come excessively close to the chain.