Hi CoDash, and welcome to the forum. I see you're in VA. If you're talking about securing a firearm in the lock box while in Virginia, then the lock box doesn't "have" to be anything. There is no law in VA that requires you to lock the firearm. You're allowed under VA law (18.2-308 section 10) to have a loaded firearm in your vehicle in a "secure container". That container does not have to be locked by law.
If you're talking about *transporting* an unloaded firearm from one place where you can legally posses the firearm to another legal place, but through a gun unfriendly jurisdiction (DC, MD, etc.), that's a different matter. Federal law does require a locked container. If that's the case let me know and we can go down that path.
It sounds like you're just looking to secure the firearm in your truck to prevent theft. The box you picked out looks like it would be fine. Another manufacturer of these kind of safes is GunVault http://www.gunvault.com/. Another company that makes lock boxes for trucks is TruckVault http://www.truckvault.com. Their boxes for trucks appear to be intended for the truck bed, but they might have something for under the seat. Yet another company is BedBunker (www.bedgunsafe.com). They make safes that replace the box spring on your bed, but also make a lock box that fits under the seat of a pick up truck. Check this out at: http://bedgunsafe.com/truck-bunker
Hope this is of some help.