I took this to the park this morning, it survived the journey in my backpack but it exposed a dodgy RCA connector. The case I used (temporarily) for it is a cylindrical tea carton - the red 'tower' behind. Battery power comes from 3 NiMH AA cells giving around 10hrs of listening.
I've just done a mental estimate of the BOM cost of this DAC (labour cost is way dominant, but this is DIY right?) and it turns out to be dominated by the cost of the caps in the top three tiers (the small black ones). These are real Japanese Rubycons, chosen for their diminutive height (16mm) vs the Chinese ones at 25mm height. So if you didn't mind adding 3cm to the overall neight you could build this with all Chinese caps and save a few dollars. As it is the total parts cost is under $30 including batteries. If you wanted S/PDIF input (currently its I2S) you'd add a couple more $.