I’ve made several such ‘concrete’ boxes over the years, as I really enjoy having concise instruments, rather than a bag of loose bits. My best suggestion is to start exploring with two different bases at the same time: one a simple length of 2x4" lumber (~2 to 3ft long) and, if you can manage to find one, a repurposed hardwood box (old jewelry boxes work well). Lighter woods such as basswood, cigar boxes, or crafting plywoods, are challenging as they tend to sound very thin and hollow, and so does the sound transmitted through them. Heavier/denser woods tend to transmit a wider frequency range, which is a particularly nice starting point when you’re using piezo mics. It is a good idea to isolate these bases from work tables with a thin layer of foam or synthetic felt, so you avoid unwanted surface feedback/transference, especially if you’re working in spaces with larger speakers, or other musicians.
Once you have these two bases in front of you, explore different positions for the piezo mic (again if that’s the route you’re going versus directional mics) to see where it sounds best on both, just by rubbing or tapping with your hand or using random kitchen utensils. The choice of mic amplification will also play a role. It really depends on your personal sonic goals…these homemade instruments can sound as clean and detailed as an acoustic guitar, or as dense and overdriven as you want, with the right choice of mic, and/or addition of effects (pedals, or DAW-based).
From this point forward, it is exploring what different materials and objects sound good to your particular needs. This might be using eyelet screws with suspended wires and hanging objects (wooden clothes pins, metal washers, etc), or varied textural surfaces such as sandpaper or gravel that you move objects across. Contrasting approaches offers more flexibility than a tight group of similar objects and materials - which can be difficult to appreciate and distinguish in a resulting recording.
I don’t think there is a better way forward than using your own ears, a microphone, and a pair of headphones to explore what moves you in this particular mode of instrument making. The lack of specific shared end-point means that it can be whatever you need it to look/sound/feel like, whether it is a 2ft piece of lumber with a handful of things screwed to it, or a giant plastic sphere filled with loose objects clanking together.
