I havnt done anything minecraft for a while, but you might be able to teleport the chest from one spot to another continuously, so that it appears to stay there, are maybe just place the items from the one you see into the one you dont.

I have an idea about this, think i might get back onto minecraft

Fill clocks trigger commandblocks 20 times a second, should be fast enough.

Well, you said this will be multiplayer. You can do this with Worldguard. Just make a region that includes just the chest and set a flag for the region. I believe the flags "tnt" "other-explosions" and "creeper-explosions" will do it if they are all set to deny.

maybe put a command block under wood or whatever you use then put bedrock around it and whatever but in a way you like but the command block is right under the chest in your way with a comparater fast pulse repeater using /kill @e[type=!Player,r=8} and changing the r=8 to r= and any other number you can customize it but if you want you can change !Player to PrimedTNT it should only get rid of tnt but !Player gets rid of all non player entitys in its radius but dont remove the r= part or it does that to everything not that area

i did a fail lol i edit the code

Maybe try using an ender chest and finding some way to make it accesible to all players. Could you put a sign on the front and do "[private]
(user's name)
[more users]
everyone
Try that maybe...
I haven't tested it X3

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Yeah This is Possible. But I'm Not Makeing The Command!

Is this is single player or multiplayer?