Has anyone tried doing something like this?

Yes many times . Lets discuss the details.

https://www.amazon.co.uk/Amtech-H0500-Number-Letter-36-Piece/dp/B0039UIGXQ/ref=sr_1_3?keywords=letter+punch+metal&qid=1572263695&s=kitchen&sr=1-3

Is not intended to punch steel but brass or copper (do not use aluminum as that will distort and warp in fires). See from the comments itself like this one -

"Wouldn't even touch a peice of mild steel, punch bounced off and flew across workshop leaving no mark other than flattening the punch letter itself"

Here is what I would suggest instead -

https://www.amazon.com/Safe-Seed-Recovery-Passphrase-CryptoCurrency/dp/B07CLMK3WJ/ref=sr_1_14?keywords=bitcoin+metal+wallet&qid=1572267413&sr=8-14

and 1/8 or 4mm at the largest stamp kit

https://www.amazon.com/ABN-Metal-Small-Stamping-36-Piece/dp/B01MZ6HYQZ/ref=sr_1_6?keywords=1%2F8+stamp+set&qid=1572267596&sr=8-6

Now you may be thinking that Its easiest to preset the words before hand and place them in a vice, mold, or tape them together and than use a hammer to smack the whole word at the same time but you would be wrong due to

1) It is much better you hit the stamps with a large amount of force the first time, and the more your repeat the more chance it will slightly shift leading to double/tipple stamped letters (slightly blurry double vision result )

2) The letters are tricky to position the right way in a set because they are mirror images of what you intend to stamp and easier to do one at a time . Doing the letters one at a time seems like it would take longer but is quicker in reality

3) The spacing is too much even if you have them side by side unless you do them one at a time

P.S... the 6 and 9 is the same bit in these sets so you just reuse that

P.S.S. technically you only need to stamp out the first 4 letters in BIP 39 backups as those are always unique , but with the 1/8 set you can stamp out the whole word so it looks better with the size of that metal plate above

Now I see amazon for some reason doesn't list safe seed in their UK region so you can simply look for -

100mm x 150mm x 0.9 mm copper

https://www.amazon.co.uk/Metal-Off-Cuts-Quality-Copper/dp/B00BGAW9XE/ref=pd_sbs_60_5/258-7000259-1804913?_encoding=UTF8&pd_rd_i=B00BGAWC2W&pd_rd_r=391d8b9d-4ae9-491c-9189-dd8594da9b37&pd_rd_w=krgPh&pd_rd_wg=9BdC8&pf_rd_p=cc188cba-1892-42b3-956f-6c67d0ab7a00&pf_rd_r=SWYE47GPQFKV044RWETE&refRID=SWYE47GPQFKV044RWETE&th=1

Since this is larger you can use a 3/8" stamp kit or 5mm instead of smaller one as well

Other tips -

1) Never stamp your passphrase (13th / 25th word on the same metal plate as the 12 /24 seed words) . When using the passphrase feature found in certain wallets you want the passphrase to be recorded on paper or preferably metal separate than the backup 12/24 words for better security (you are essentially making a 2 of 2 multisig)

https://wiki.trezor.io/Passphrase

https://support.ledger.com/hc/en-us/articles/115005214529-Advanced-passphrase-security

https://coldcardwallet.com/docs/passphrase

2) In the extra field stamp the derivation address used in your wallet or at least the wallet name that you used to create the private keys .

Here is an excellent site telling you all the default derivation paths

https://walletsrecovery.org

3) Place the brass or copper on some soft wood while stamping it to absorb the impact and allow for better stamps

Here is a detail review of other metal backup wallets -

https://medium.com/@lopp/metal-bitcoin-seed-storage-stress-test-21f47cf8e6f5

https://medium.com/@lopp/metal-bitcoin-seed-storage-stress-test-part-ii-d309e04aefeb