I've used WeBoost devices on mobile equipment trailers, works fine but you need to have the outdoor antenna outside. They aren't cheap.
I used to work for WeBoost (Wilson Electronics, St. George, UT) The boosters work well but you need decent signal in the first place to boost it effectively. Get one with an omni-directional antenna if you do decide to purchase. FYI - They assemble and test them here in the USA (with a lot of mexican and temp labor) and source component parts from overseas. (china).
I had a cheap one for t-mobile years ago. It worked "well enough" but left a lot to be desired. Currently I have WiFi calling too so as long as my wifi network is up and accessible I have service. That could be another alternative but it would require a wifi signal in the box.
Wifi isn't an option. My workaround now is to start my phone hotspot and place it on an interior cross bar of the overhead door. The signal penetrates the door a little bit. Then I use an old phone with wifi. Sketchy and not reliable.
I've used the weBoost in the pickup. Works as advertised. I'm often out of cell range or in spotty areas and it definitely improves the quality of signal in those areas. Idk how much it'll disperse across a larger area though. Works fine for the vehicle though.
Thanks! The space is only 30x40. It's actually a storage condo, so there are multiple walls of other units on both sides.
I installed one on my house, it works well but the one I have has pretty limited range on the interior antenna though that's because I installed a mobile one. I have effectively zero signal outside and get 2-3 bars near the repeater.