WIF keys are mixed-case alphanumeric. One thing you could do (shamelessly take from yeticold) is convert to cased nato phoenetic alphabet. Lower-cased letter is all lower-case, capital letter is all upper-case, numbers are spelled out (2 == TWO). For example here's an hdseed from a regtest wallet:

cPzhezEaMfyxxZTA8iALDWAyHs5T62hKcwmkQJ6VT8GMX2QWgEaQ

it becomes

charlie PAPA zulu hotel echo zulu ECHO .... ECHO alpha QUEBEC

You have less density than bip39 (you will have one word per character). So it's a lot to write, but you get a lot of built in error correction (if the ink is smudged you can still distinguish alpha from bravo, etc) and it doesnt have the "shared prefix" issues that the bip39 list does. So less density, more readability. Another nice thing is you can just do it with pen and paper and no wordlist.

Just an idea.