Note: updated BOM
Added extra 1/4" aluminum spacers to the BOM, they are needed for the two plate setup (two per wheel) and the mini V wheel kit only comes with one per wheel. Alternatively you could easily print this item. Just print them in multiples of 4 so they are identical in height.

 

If it wasn't for shipping the open build cost wouldn't be too bad. I have found that the shipping estimator is bad for small packages, but if you contact the staff they can get you a better rate.
Here is a printable replacement.
http://www.thingiverse.com/thing:502734
Alternately, something like this plus a few M5 washers might do the trick, need to get to around 1/4" or 6.35mm.
http://www.ebay.ca/itm/50x-5mm-Nylo...849?pt=LH_DefaultDomain_0&hash=item1e76411221

 

nice find, has a good wall thickness to prevent it from cutting into the plastic upon tightening.

 

Click on "files", the tab to the right of the "parts list" tab that you are on in the picture above. It is mixed in with the stl files. There are two copies; a xls and a xlsx. I tried putting it in the parts list area but couldn't get it to appear.

 

I took a few pics of my extrusion for a visual. It may be an optical illusion, but I still think the extrusion inside corner fillet are too big a radius.

Something I like to do to help my slicer (slic3r) print holes in a bridged section, is to put a 0.2mm layer at the bottom of the hole so that the first layer bridges right across the hole. You may want to consider doing this for the two holes shown below.

What is the width "w" in the pic above? If the idlers clear this then no concern, but if not I wanted to make sure there was enough room for a M3 washer between the to idler bearings, a width of about 8.5mm.

 

If memory servers me correctly: I printed mine in PLA with a 0.4mm nozzle, 0.2mm layers, 6-8 forced perimeters, 10-12 top/bottom layers, and 40% honeycomb infill.

I've set the BOM to use all 20x40 extrusion. Certain pieces can be substituted with 20x20 if desired such as the lower 3 horizontals (not the Z motor rail) and the front top horizontal piece. You can see I used two pieces of 20x20 as I was short on 20x40 at the time.

Exactly, the extrusion requirements will vary with build size. Open builds offers lengths in both 1m and 1.5m lengths, so you will have to do some more detailed math then what is done in the excel sheet to order the minimum required and reduce wastage.
You can see from the picture of my printer that I gave a certain amount of room beyond the print bed. I am hoping it will allow dual extrusion in the future as well as a print cooling fan for printing PLA. Since I haven't made these accessories yet these lengths aren't optimized. I did the same with the Z lead screw length as there are different lengths of hotends out there. Feel free to increase or decease these amounts if you want more room or a tighter build.

 

Completely up to you! I sourced 300mm lead screws so that is what I used and it is a fairly typical build height for reprap printers, excluding deltas. Open builds offers longer lengths of lead screws so feel free to modify this value in the sheet.

 

My latest print:
http://www.thingiverse.com/thing:16218

 

I don't have the exact hours as I turned of the power before checking. I'd have to say about 20hours. It ran overnight and ended some time while I was at work. I left it at default speeds 30/60/130.
I am using the latest release of slic3r. I've found that pillar support works well and breaks away fairly easy.

 

I've seen others that have used a similar setup with smooth rod and linear bearings. It seems that they don't have any issues, but my concern is that there is a risk of slight teetering left and right. The actual forces involved would be quite small if everything is balanced, but an unbalance can also create a constant predictable force direction. You might get away with only eight V-wheels, either all inside or outside the extrusion.
Only speculation.
The alternative would he a heavy duty redesign of the current Z arrangement to full size solid V wheels, dual plate setup, and 20x60 or even 20X80 extrusion.