I made a more rigid corner brace than what I've seen on Thingiverse so far.
(4/8/2018 Update: I finished adding these supports to my own MPCNC and the frame is as rock solid as my table. I think it was worth the effort.
The only problem now is Z wobble. not sure how to tackle that yet.

• Corner Brace
  23.5, 24.5, 25, 25.4mm versions are now available.

  12x / M3 x 16mm screws

• Angled Mount (For old flex foot)
  The angle mount currently requires you to print a flex foot for each one.
  I did it this way to make it compatible with everyone's size. I may eventually make a one-piece angled foot to save print time, and reduce the number of parts.
  The angle bracket foot has holes for both M3 and M4 bolts for attaching the flex foot you print.

  Screws
  16x / M3 x 12mm screws. used to bolt to the foot. (can change to other sizes if requested.)
  https://tinyurl.com/y8a58a5l <-- for angle mount and corner braces.
  32x wood/drywall screws needed to attach to the table (use what you used for your 4 main feet)

• Angled mount (For new Foot 2018)
  Compatible with the new foot uploaded to the Main MPCNC on Mar 19, 2018 by Allted.
  Uses same slotted holes to attach to your table. Requires one bolt or screw to be angled, sorry if that's an issue.

  Screws
  24x / M5 x16 screws can be used to bolt to the foot.
  M5 Screws https://tinyurl.com/y8h62f6d
  M5 Nuts https://tinyurl.com/yc83nu9e
  32x wood/drywall screws needed to attach to the table (use what you used for your 4 main feet)

• Support Leg Length Calculator
  (ATTENTION JsCalc is going premium and i cant afford $10 a month for a calc i dont use
  so i will add a Dropbox link for the script and add the algorithm here.)

Metric: https://jscalc.io/calc/mvBBqJ1AphGDq2ax
Imperial: https://jscalc.io/calc/JhahDCAHDxAwHanc
Dropbox JS file: https://www.dropbox.com/s/ya71hzpe0p4px52/jsCalc.txt?dl=0

Enter your z height to get "Support Leg" length
if you want it lower subtract that distance from your z and use that in the input.

Algorithm:
It uses a modified Pythagorean Theorem with offsets to account for both ends of the brackets. only showing metric you can convert if using imperial.

Metric:
Z = Z height
(((Z - 72.5)^2)*2) - 13

(Example: 200mmZ - 72.5 = 127.5,
127.5^2 = 16256.25,
16256.25 * 2 = 32512.5, (double to achieve 45deg angle)
√ 32512.5 = 180.3,
180.3 - 13 = 167.3mm)