I would just make room be able to write a WIF privKey, but if you really have to use a shorter format, you can use a "
mini private key".
If you're using Windows, casascius had a compiled binary of his
"Bitcoin-Address-Utility" repository on his website but was later removed.
You can still access it though through archive.org's server:
web.archive.org/web/20210801000000*/http://casascius.com/btcaddress-alpha.zipTo get the file: Select a year (
pref 2021), point to a highlighted date (
April 16) and click the time of the snapshot.
That will lead you to the archived url which is the copy of the old download link.
After downloading "
btcaddress-alpha.zip", extract it.
Run "
BtcAddress.exe", create a Minikey in "
Tools->Address Utility".
Note: Mini Private keys aren't compatible with some wallets. AFAIK, Electrum supports it.
For Linux, compile it yourself.