Beyond the security rating it would be nice to have something with some decent capacity. The stuff that's out there seems to have 20 cu ft or less of room.
View Quote
Generally speaking, that's because the things that usually require this type of security don't take up much room. That and weight. A TL-30 is usually insurable up into the mid 6 figures. Most people using them are storing cash, metals, or jewelry/stones. Most do not have guns worth that much.
That Super Diamond I told you I have weighs 8,500 pounds. That's not going into your typical home.
You can pick up a used composite TL rated safe, large enough for guns, for $2,500 to $4,500 on average. Safes built by those who are recognized by some of the most renowned insurance providers in the world. Very large safes are uncommon, and the few that are made would be very pricey. You're essentially going to get into modular vault territory before too long. I have a painter. I have a woodworker. I have an upholsterer. Any safe can be a "gun safe".
If you require good security I'd start there. A good safe. Then if you want to store your gun collection, adapt the good safe you found for that purpose.