Something like that should do it for you.

You need that sort of thing +/- maybe a little piece of bent metal extension to create a nub from the your seat post just above the BB to let the bottom/front of the rear fender attach on, replacing what on most bikes is the little bridge between the chain stays.

You will probably need to reposition the brake bridge tab on the fender. Amazing how good a little pop rivet is for that.

The P clamps already mentioned will let you connect the fender stays to your chain stays down near the rear hub.

That gives you support front, middle-ish (a little too far forward, but OK) and rear-ish.

Good luck.