Your basic concept looks reasonable, but I think using wood near the water line is asking for trouble. Wood fibers tends to swell and distort with changes in humidity.
But to be brutally honest, do you have any idea how much work it is to clean up water damage? It's a nightmare!
My LFS has 75's on sale for 85 bucks
! I really think this is "false economy" and bordering on Russian roulette. If you were just using this tank in the garage as a minnow live well, I'd say "Go for it, who cares if the thing explodes!" But inside a home, this is a gamble I wouldn't take... But that's just me. I mean for less than the cost of a good canister filter you can replace the entire tank: I'd just do that rather than risking a repair job.
Be sure to post pics if you attempt the repair!