Now that I'm enjoying my synth computer-less, what's a good way to get some stereo delay and reverb, and also a stereo headphone jack output? (I'm not using speakers, so 1/4" outputs are not needed).

unless you really want to commit to eurorack (which, be warned, may tie up all your disposable income for years) then you may be better off with a smallish mixer that has built in digital effects - it's possible that one of these may also work as an audio interface, which would be a bonus

I was thinking maybe an effects module (such as Mutable Instruments Beads/Clouds) plus an "ALM019 - HPO" could do the trick.

modular effects rack = modular synth where the sound source is external

still need to think about modulation sources and utilities... so make sure you leave space to expand into in the case whatever you buy...

clouds cries out for at least a couple of channels of modulation sources - possibly one or more lfos plus random or chaotic - and attenuators or attenuverters as a minimum

beads has attenurandomisers built in, but would still benefit from lfos

I could make my own wooden box for these, but then I'd need a power supply. Should I get a eurorack power supply/bus and just use that, further complicating something seemingly small? I'm not yet ready to take the Model D out if its box, but perhaps I could tap it's power supply?

wooden boxes are easy - buy rails & threaded inserts though - they make life much easier & make bigger than you think you'll need - 6u 1-4hp is a good starter size

& yes you'll need a power supply - I like befaco excalibus which is available built or as a kit

as the model d has a powered case already I'd keep it in there... taking it out ans putting it in a larger case is a waste

"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia

Utility modules are the dull polish that makes the shiny modules actually shine!!!

sound sources < sound modifiers < modulation sources < utilities