It's actually a far easier process than most people might think. The real key is in figuring out the dimensions of the rails (and which rails) and mounting plates. Given that all of these are easily available from various sources, with dimensions clearly given by the manufacturers, it's simply a matter of sorting out how much hp you want, how many rows (and what type rows), and then proceeding from there. But let this stage be the actual startpoint...work from the inside out!
As for the outside, that's up to you. You can go with some sort of hi-impact paint to cover, or source up some tolex in various patterns. Alligator? Sure, why not? One thing about the outside that I suggest, though, is to find quality ATA case fitments and use them...corners, edge rails, handles, closures. They will up the weight...but if you plan to use the case on gigs, you'll be very grateful for those. They also look pretty professional in a studio setting.
Third (and most important!) consideration: power. You should fit this with a power supply that, optimally, provides around 1/3rd more amperage than your system's total draw. There are two reasons for this...first, less stress on the power supply means less heat and less wear on its components. Heat, also, might be a factor in detuning modules, so making the P/S hot as a firecracker is a bad thing for stability. And the second point is current inrush; for less than a second after switching the system on, the total current draw will be somewhat higher than the 'in operation' figures. Best rule of thumb says that with 100% solid state components, around a 1/3rd surplus should more than handle inrush. But if there are tube components, their inrush and warmup stages can actually close to double their current draw for a few hundred milliseconds and that'll have to be taken into account in the P/S spec. As for the AC connection, use a standard fused IEC female chassis mount and mount the P/S as close as you can to this to minimize the AC line run within the cab. Remember: those lines can induce plenty of hum, so keeping them to an absolute minimum length inside your synth is critical.
Most systems these days with inboard power use switching supplies. And if you're careful to choose reputable makes of supplies, this can be OK. But with switching supplies, you have a risk of ultrasonic AC ripple creeping onto your DC bus lines. To avoid prolems with this, there are two strategies. First, don't use a switching supply; linear DC supplies don't have these problems with ripple or noise to an extent anywhere similar to switching supplies. So you win on stability, but linear supplies are VERY heavy, clunky things and take up more room inside your cab than switchers. But if you can swing it, or opt for an external cab for your P/S with a beefy polarized multipin or Anderson PowerPoles to connect your cab's DC busses to the supply, they are the optimal choice electronically. The other strategy is to use Eurorack power bus board with filtering. And actually, you should do this anyway as the filtering schemes not only deal with a lot of the garbage on the main DC busses, but they quiet down any crosstalk coming back down the DC lines from the modules. Some bus boards these days also have onboard 5V supplies...and if you have modules that require it, these are pretty goofproof and effective ways of supplying that.
Your power bus lines inside the cab should be fairly heavy gauge, because this also helps with heating issues (some of which could lead to failure) and eases any impedance load on your P/S. Using stranded 12 or 14 ga copper wiring might seem like overkill, but I recommend it due to both the current loads being carried on these lines and the impedance benefit. Make sure your DC bus lines are well-secured to the inside of the case, using the shortest runs necessary from the P/S to the busboards. For DC distribution and any inline interconnections, I strongly recommend automotive-grade distro blocks and connectors, as these are very beefy, designed to deal with high temps and vibration, and their design is overspecced for your use. BTW, the more you can OVERspec your power system, the better (and more reliably) the build will work!
Your bus boards should be on spacers to keep them away from the rear wall of the cab. 1/4" clearance is good, and of course, higher is better. The point of these is to help prevent anything that might get loose in the case from contacting the bus board traces and shorting the power supply. And also, making sure your individual DC lines coming off the P/S have individual fusing per line is another good safety idea. Don't skimp on cost on these boards, either; they are, quite literally, the 'backbone' of your synth. Some of the newer boards with wide/thick power bus traces are definitely not about the hype...they'll handle current better and, natch, ease the impedance burden on the P/S a bit more.
A lot of these design points actually come from my own 22 module Digisound 80, which resides in a custom case with a power system designed and built by the late, great Kevin Lightner. I've had this for a couple of decades now...and it's a 100% solid performer. The dude built his custom designs to last forever, and his logic behind the choices that made that happen was totally bulletproof. Other bits are from my amateur radio experience, where high-amperage DC is a common thing.