Update Rev5 2/11/2016
I corrected a mirror problem that pgx3d spotted. I also added 8" & 12 test parts & updated them so the bottom side of them should line up with the back of the Y-axis Wood Plate.

While working on a plastic design of the Y-axis backplate for the i3v this idea came about after talking with my friend voodoo28 and his problem with the 12" version flexing. I like making parametric designs, so I went ahead & made this design so you could make a brace for either the 8", 10" or 12" i3v. I have the 10" version and have not experienced this problem. this is probably only needed for the 12" version & is untested, but seems like it should work.

If you are interested in trying this, I would print the test stl file for the i3v version you are using to make sure the bolt holes line up 1st, before printing main part. I tested the 10" version & it lines up for that. It took me 33 minutes to print the test part for the 10" using 20%infill & 2 solid layers for top & bottom. The final part with 50% infill would probably take about 2 hours to print. The test prints have the holes the diameter of the bolt heads. That way the test print should fit flat against the back of the y-axis plate if they are lined up right. The dimensions for the 8" i3v I got from Colin, so they should also be correct. The 12" version I got from:
https://github.com/DonKahones/MakerFarm-Prusa-i3v-12-Inch

I included a test stl for each i3v version, and final stl file for each i3v as well as an openscad file you can change to make it a thicker or thinner brace & bigger or smaller ledge in the middle. There are 5 variables specific to which i3v you are using, just comment out the variables the versions you are not using.

Here are the ones for the 12" version
i3v = 12;
i3v_Width = 346.5; //12" i3v = 346.5mm
i3v_Vslot_Width = 238.8; //12" i3v = 238.68 or 238.8
i3v_Vslot_Offset = 63.5; //12" i3v = 63.5
i3v_MotorOffset = 166.6; //12" i3v = 166.6

This brace in its current size is 6mm thick & 2mm countersink for bolts, so your bolts will have to be 4mm longer. That is you will have to replace the current two M3x16 bolts with M3x20 & the two M5x12 bolts with M5x16.