This is an important question. You said it doesn't have to be exact, but how inexact is good enough?

You can take a permanent marker, write 160 w on a piece of tape, and stick it on your stem for a lot less than a new cyclocomputer. Total cost = $0.00 assuming you already have the parts.

If you have a Garmin or a smart phone, you can get free power guesses from Strava or Golden Cheetah.

Another option is you can spend maybe $100 or more on a computer that will give you a different wrong guess, and change it more often.