How much extra weight is added by strengthening a piston-prop fighter for carrier landings? To land on a carrier, an aircraft must be strong enough to withstand sudden deceleration by the tail hook catching the arresting wire. I think it also requires strengthening the undercarriage to survive a harder "plop" on landing.

So I would like to know, how much extra weight is added due to both of these things?

I'm interested in the WW2-era piston props. The modern jet fighters are much heaver, so I'd rather save that for a later question.

Ideally, the greatest example would be a piston-prop from this era made in two versions: land and naval. Then we could just compare loaded weights. But I can't think of any. Don't think I've ever heard of that. All of the carrier-based aircraft from WW2 that I know of, were purpose built for carriers and never had a land version. Hopefully someone knows where to find airframe specs for these aircraft and has the engineering know-how to say how much steel would be unnecessary for a land version.
The British adopted the Spitfire and Hurricane for carrier use.
Using the Spitfire as an example, here's data from Wikipedia.

Spitfire Mk.VB is 5065lbs clean.

Seafire Mk.III is 6204lbs clean.

They're not exactly equivalent otherwise, but close enough. 
So the Seafire is roughly 1140lbs heavier than the land based equivalent, or over 20%.

Of course this will vary wildly for different variants compared, but it's a ballpark figure