Every figure collection at some point needs a guitar for their accessory pool. It's one of the unspoken rules of collecting idolatrous representations of paganistic figureheads, er, I mean, toys. Your shelf's awesome quota gets fulfilled by 3% with each guitar with a ceiling of 15% maximum added value.
I actually just wanted a guitar for my Engineer. It's way better than the Gunslinger since I don't have to amputate an already half assed glove sculpt and make reinforcements to make it work. More of also just wanted the Engineer to be able to do the taunt kill with a guitar.
First, like a good Engineer, we'll need some schematics. Unfortunately, I couldn't be bothered drafting up any, so I did the next best thing: have someone else do it for me. Here's a great set of schematics that I used to build my guitar, courtesy of Christophe Grellier's guitar site. I scaled down his drawing to 69.5% to achieve 1:6 scale.
Ronom Unicorns and Surging Sentinels were sacrificed to achieve the task. I split the cards into 15mm wide strips, then arranged them inside the stencil 5 layers thick. Use tape to hold the walls next to the stencil as you layer the insides with more cards. I ended up using 5 layers instead of 4 because I had excess material from cutting up 4 cards. More is better anyways.