GLS. Inexpensive, fully repairable, fully modifiable.
However ... I use Dot Com cables inside of racks ... like for connecting various rack gear to patch bays. The Dot Com cables (the regular ones, not the new skinny ones) are built well and are reliable. Inside of a densely cabled rack, I can't think of a more important place to use good cables. Who wants to chase down cable issues inside of a 16U rack that has two dozen patch cables inside of it? Y'know? So I use a lot of the 1' ~brown~ Dot Com cables inside of my racks. I have this one rack module ... it's an Alesis DEQ830 digital graphic EQ ... it has eight inputs and eight outputs all wedged into a 1U enclosure. It's patched up to a patchbay to provide access to all of the I/O jacks mo' bettah like. So sixteen of the 1' Dot Com cables to the rescue.
Back to the GLS cables, they are super flexible and really cost effective ... easily repaired to boot. Purty colors!
On that, 5U ... The End. I'm going to bang my drum on power systems though ... design/fab the power system (PSU/distribution) FIRST .... then design/fab the cabinet around the system SECOND. And for all that is good ... please (please!) use a really solid linear power supply, and consider using/buying/making a bus bar distribution system. Take into consideration the total monetary value of the entire modular synth ... is it not worth spending at least a measly 10% of the modules' total value on providing them with "good power"? So many (too many) people take the power system as an afterthought. But if you go integrating digital modules along with analog modules, as well as high falootin' tricked out control goodies your system as a whole will NEED solid power. Ok, done finger waggin' on that (at least in this thread).
I'm going to break ranks here and suggest using a small contingent of Euro, just for modulation/controller purposes. As I have said before, there is N.O.T.H.I.N.G. like the Make Noise MATHS in 5U ... anywhere, by anyone (I have FOUR
of them within my own 5U system). There are also some really useful and FUN! real time controllers in Euro that are not available as 5U protocols. Those Soundmachine Lightstrips and Lightplane are a blast to use ... the Lightstrips record up to 9 seconds of action, and may by ~triggered~ by an incoming gate signal. So you record your modulation actions into the Lightstrip, then a gate signal may be used to "fire off" that recorded CV modulation, thereby providing clock-sync'd resets. There are also some really nice joysticks in Euro by the likes of Flight of Harmony ("Choices" stick), and Befaco (oh hell ... here it comes! .... BEEF TACO!!!!!!!!!!!!!! ... dammit .. happens every time ... kinda like the cartoon strip about an ex-pro football player-turned-sportscaster "Tank McNamara" anytime someone yells "FUMBLE!"). ~ahem~ ... moving forward, Befaco makes some really killa envelope generators that fire off a gate at the end of each ADSR stage (how bitchin' is THAT?). There are also some really fun little sequencer toys, and a few outstandingly great digital/wavetable LFOs ... the Synthesis Technology E355 comes to mind (just bought one myself!).
So maybe a single 84HP row of Euro might be a good 5U extension to consider.
Remember ... power system first, cabinet second.