Finally starting to look like a printer!
Best of luck!
@Mark_Zimmerman RepRapPro 140x140 heatbed inbound shortly, first I just need to redesign my extruder mounting bracket!
Looks a lot like some of my early prototypes for the Bukito. One thing you may want to consider is that those idlers aren’t going to want to maintain their angle with tension on the belt. I originally used two mounting screws to hold each piece for this reason, but in the end I replaced one with an alignment tab that sat in the slot to keep it from rotating around the single mounting screw.
It also looks like you’re unnecessarily limiting your travel by mounting the idler to the front of the beam. Ours is mounted only to the back, and the carriage can go right off the end if we want it to. Combined with the off-center mounting of the hot end, the print area reaches almost all the way to the right-most limit of the machine. I also paid attention to clearance on the back side of the carriage so that it doesn’t hit the X/Z interface piece, and it can be moved completely in front of the Z beam. I estimate that all this makes the X beam about 40mm shorter than you would need with this design or the Ormerod’s to get the amount of X travel.
Oh, another advantage of having only one screw locking the idler piece to the beam is that it’s SUPER easy to tension the belts. Just pull on the idler until it’s tight with one hand, then tighten the screw with the other hand to lock it in place.
@Whosa_whatsis Thanks for the comments, I’ll try to answer your concerns in order 
Firstly X-axis idler is held on the rear by 2 fixings ensuring the pulley doesn’t rotate up or down, the front one fixing bracket (one you can see) is only there to ensure the pulley doesn’t flex in towards the X-axis V-slot with belt tension. All seems pretty secure tensioned up.
Secondly if you look at the X-carriage (pictured in another post) you can see that it is shaped to avoid the front one fixing idler support, so the carriage goes all the way to the end. My carriage reaches all the way to the end of the 140mm heat bed. Also the carriage travels all the way to the GT2 pulley as you mentioned yours does by clearing the X/Z- axis. So basically not 1mm of beam wasted! Also I kept my hot end centralised to the pulleys to ensure it stayed stable.
Thirdly I’ve made the belt tension-able at both the motor end and the idler end, hence all the excess holes in the second X/Z-axis plate for allen keys. As you mentioned you can extend the idler when in position or drive the X-axis off the top of the Z-axis to tighten the motor end if idler at max.
I will see if I can add some more pictures to show what I mean. Amazing how rigid the whole OpenBuilds V-slot system is!