According to iFIxit, the tall, rectangular device is a series of "vertical circuit boards surrounding a big empty space." This space is big enough to hold a 3.5-inch hard drive, but iFIxit needs to look at the new Time Capsule to determine whether a DIYer could save some cash by purchasing an AirPort Extreme and installing their own hard drive into the device.
Good news on the repair front as the AirPort Extreme A1521 is relatively easy to crack open and repair. The unit was given a Repairability Score of 8 out of 10 with a few points lost because you have to disassemble most of the device to clean the fan. Some of the connectors are also small and could easily break, iFixit added.
You can read more about the 2013 AirPort Extreme teardown on iFixit's website.