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[peteyb]

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[back40rebel]

I think they have some keyed patio door locks that you could put on the 2 end doors. Maybe put one inside & the other on the outside. You might have to channel your inner Macgyver

Amazon Link

 

[96cbr900rr]

I used a hasp type with built in lock, of course the goal was only to keep the kids out when they were little.

http://www.amazon.com/gp/aw/d/B00004Z0ZH/ref=redir_mdp_mobile/183-6069704-5442904

 

[MA_Shooter]

Here are some options-

http://www.thehardwarehut.com/catalog-product.php?p_ref=17950

http://www.thehardwarehut.com/catalog-product.php?p_ref=17949

You might consider putting a sliding bolt on one of the doors that will anchor it to the floor or upper door jam.

 

[swampy]

A locked closet door is a dead give away that there is something valuable inside. You'd be better off bolting a small Stackon cabinet to the floor or wall for the ammo and putting something in front of it so if a baddie peeks in the closet all he'll see is your prom dress and ed hardy t-shirt collection.

 

[Scott D.]

Remove the bi-folds and install a double door.

 

[tbk5]

And another...
http://www.mrlock.com/23315.html

 

[F2blica]

Remove the bi-folds and install a double door.

This