I have a space in the corner of my garage that measures 68 1/2" x 8 FT x 27 1/2"
I am looking for ideas to turn this space into a lockable hidden storage space.
My current idea is to frame the space in 2xs and add a locking steel door. Then cover that with an outer storage slatwall.
So hidden without making it overly hard to access.
I know there are enough builders and carpenters that would have some other genius ideas?

Hidden, lockable, easy to access, storage space...

What are you storing?

If guns and valuables id probably just buy a good gun safe and bolt it down

But if this is for spray paint and wd40 that may be overkill

Pictures and what you envision storing in it would be helpful. Disguising with shelving or refrigerator door comes to mind - 2 very common things seen in a garage that thieves would walk right by.

The other thing to keep in mind when talking about space in a garage is climate control. You will need to insulate it well and put some sort of humidity control in the space.

Another thing to think about is IF someone wanted to get into that space, how long would the wooden framed fiberboard hold up to a sledgehammer or crowbar?

Even the best gun safes are not “proof” that someone, who is motivated and had the time, could not get into. I would think you need to either make it really hidden, or go the safe route and bolt into concrete floor.

I'm not under any illusion that this would be any kind of extreme security.
And I have seen high dollar safes peeled open like a tuna can from the sides.
As you say any one with determination will get in. I am wanting a space that might house some of my nicer pieces that tend to get bumped around in the safe.
It may house some other items that can be replaced but are of logistical value.
I can add dehumidification and lighting to finish it off.
I was looking for ideas to camouflage the exterior.
My next house will have an out building with a vault !

A great way to camouflage it would be to screw a bookshelf to the door such that just the hinges are visible. Put the door handle on one of the shelves. Just attach it and open it when you want to enter.