If it's all for fun then great.

Something to consider. If you just need to make the plane so that it'll bleed off speed faster than the drag brake under the fuselage will work. But the model will still stall at the same airspeed and so it'll still be landing at the same airspeed as before. If you want it to actually fly slower then you need the extra lift provided by proper flaps that add both drag AND lift.

I also think you'll find that a longer and relatively narrow flap as you're proposing will require more of a deployment angle to get a decent amount of drag. How effective it is will be based on the amount of extra sq inches you add to the frontal area. You may want to consider making it so it comes down about 60 to 70 degrees and is long enough that it comes down to the tops of the landing gear wheels. That should let the brake be able to stay down for the whole landing.

And since it's not actually on the wing there may be some trim change but nothing like if it was actuallly part of the wing's airfoil so you'll likely find that you won't need much, if any, elevator trim.

I'm reminded of the late 70's vintage FAI F3B class gliders when they first started using ailerons. At that time they didn't use flaps but did put the nose canopy on a servo so it would come up like an alligator's mouth (is that the one that lifts up? Or is that the crocodile?) to add drag for the landing task. By all accounts they didn't do much and folks soon went on to flaps which gave them more options.

Be sure to come back with the findings be they positive or negative. It may just help out the next guy.