The plastic parts that came with my i3 frame kit were unusable. And I wasn't able to print Prusa's original SCAD files from github very well. My other printer doesn't do long bridges well so I redesigned these parts to eliminate or at least minimize them.
I was a little concerned about using zip-ties but they seem to work well in holding the X rods and LM8UU bearings. The extruder isn't modified much from the original except for the placement of mounting holes. I ended up moving them around to get a good fit to the X carriage. That led to the J-Head slot mount plate's bolt hole placement being asymmetrical.
The laser cut holes for the Z rods were a bit oversized so I made a pair of supports. The Z rods are 12.5" versus the original 12" to accommodate them. Now that I think about it, I could have just printed the top header piece with a clamp built into it.
The top left X rod support needs to be carved out quite a bit If I intend to use the full volume in the Z axis. The extruder motor will hit it when it's at or near the zero position in the X axis. Maybe I should think about making a smaller extruder.
The parts were all printed at .33 mm layer height and 25% infill. I'll go over a few points about each component group:
endstops: The bigger one is for the Z stop. Use 2 x 12 mm bolts and nuts to attach a switch and 2 x 14 mm bolts and nuts to secure to the Z rod. I use the smaller bit for my Y endstop. Run a zip-tie through the shorter end and secure the block to the Y rod. Then attach the switch with a zip-tie and wrap around the Z rod. I don't need the Y to stop very precisely so the ties work great.
extruder: The standard Wade's extruder won't mount due to interference between the motor and X rods. The motor mount is elevated in this design from Prusa's github repository. I added additional support material in the bearing pocket. Use 3 x 50 mm bolts to secure to the carriage. I know. That's some long bolts.
extruder_gears: It's my biohazard gear (thing:53103) with a proper hex depth. The conical piece is used as a spacer between the spur gear and bearing. It replaces the 3~4 washers you'd normally use. Put the wider side against the spur gear. Print the washer/shim to whatever thickness you need to align the hobbed bolt properly. The gear ratio's higher than the standard Wade's at 43/13 but it doesn't stress my stepper motor at all.
hotend_fan_mount: Use 3 mm nuts and bolts to secure a 40 mm fan and 3 x 15 mm bolt to attach a J-head to the extruder.
x_carriage: There's a provision for an endstop adjustment screw. Use a 3 mm tap to form threads. Get a spring from a retractable ballpoint pen and cut it in half. Use it to apply a bit of tension to the adjustment bolt. Save the other half of the spring.
x_ends: String zip-ties from the back and secure the X rods into the U-grooves. Make it tight so the rods seat well in the groove. It's okay if the rods can slide, as long as they're not too loose. Use ties to attach LM8UU bearings to the front pieces and drop 5 mm nuts into the well for Z axis threaded rods. There should be some float in the nut wells to minimize Z axis wobble. Use a needle file to clear out the cavity if needed.
For the motor mount side, use 2 mm nuts and bolts to secure an endstop switch for the X axis limit. The motor mount holes are slotted so adjust belt tension there. Use a 3 mm tap to thread the Z stop bolt hole and use the other half of the spring to apply tension to the stop bolt.
On the idler end, break off the support material in the idler bolt hole or drill it out with a 5 mm bit. Use two 5 x 10 x 4 mm bearings and a 5 x 18 mm bolt and nut for the idler. I had some .5 mm spacers for RC cars so I used one between the bearings and another against the bearing and the top half of the X end. Print some spacers out if you don't have them lying around.
z_support: Use 3 mm nuts and bolts to clamp and 2.5 x 12 mm screws to attach to the frame.