Well, items do respawn no matter what, but theres ways to prevent that... -First you create the item you want not to dissapeer. Set it's respawn time to the max (I think 180 seconds). Then make that items run time maximum to it's top. -You create something and puts it's run time maximum to zero and it's respanwn time to 180 seconds. Now create the same item where your previous item was near to door or near the same spot you put the other item. Set it's respawn time to 180 but change place at start to no. After the first box/ item dissapeers, then the other one will take it's place (hopefully). -I haven't tested this one, but maybe you could do something with run time minimum.
maybe put a vehicle? idk. lol it's a desperate solution. just so you can get in and out every minute or so. just set vehicles to indestructible