What you are asking is basically the same as asking Vanquish or Axial for their designs, or really any company. It's not just about getting the finished design product to be able to tweak. There were countless hours of design and engineering, and revision, trial and error, etc that went into these, and the small price that I charge eventually pays for that time and effort.
I started by taking a set of calipers, and modeled the chassis and skid plate into Solidworks. Then, I designed the RULR around that part to make sure it fit well, printed a prototype, then adjusted the design, etc until I came up with the design shown. The design you see is revision 6, with many minor tweaks in the process.
Also, the material I use is different than the run-of-the-mill ABS or PLA that most 3D printers use. It's picky, finicky, and can be a PIA sometimes. (I've gone through 3 full sets of hot end components and two motherboards because of failed prints with this material before I found a good combination of settings and set-up.) The normal ABS and PLA parts are not suited to the stresses these parts will see, and a simple roll over or tumble will break the part.
I'm open to hear what tweaks you have in mind, and I do offer to make custom parts, but I will not release my designs for free. (We can talk licensing, but that is most likely not what you were thinking.)
