I need help building a flush 3×4 popup wall

I'm hosting a murder mystery party and want iron bars to pop up in front of my houses exit (roughly rebuilt) like in the second image.

I'm very new to redstone and the only solution I can think of is a cobblestone generator feed tape and that seems excessive. I don't have a ton of horizontal space to work with but I can go pretty deep and it doesn't need to be reset since it's a one time thing.

I'd really appreciate any advice!

Edit: I need it to be survival friendly, no commands