At the following site, one may obtain all parts and instructions for the delay module. 

http://store.synthrotek.com/PT2399_Dev_Delay_Kit

At the following site, one may obtain all parts and instructions for the preamp module. 

https://www.vellemanusa.com/products/view/?country=us&lang=enu&id=350480

It is advised to use metal or wood. Begin with a general design session of how you want the layout to be conceived (i.e. random, symmetric, modular-esque, etc). After a plan is sketched, locate a place for your contact mic and note this area cannot be drilled into. Begin install of springs, strings, etc. 

The PCBs should be spaced from the box appropriately such that they may be secured, but also accessible for wiring. Make sure you leave room when designing the surface. 

For the best install, I found bending the strings at a 90 degree angle and then using expanding glue to be best. It forms a solid hold on the thin pieces, allows them to resonate, and fills the hole in an aesthetically pleasing way