Hi Kiki. I have been doing this as I go along for the reasons you mention, find places where a perfect brace helps and then do what I can to make a practical brace. Here is a set of results for doing as you suggest. First, on it's own a crosswise brace under the dash does make a difference. Most of the difference goes away though when a brace from the firewall to the corner by the passenger is installed.
It looks like the car might be driveable with such a brace, you would need a short shift lever though, it would pass over that.
Here are the numbers, the lateral deflection is first and the vertical deflection second. I'll post in millemeters, just for practice and hope my countrymen don't track me down and coat me in tar and feathers...
5.76 4.70 model as posted above
4.62 4.01 model with dashboard diagonal
3.48 3.40 model with firewall thru passenger diagonal
3.81 3.56 model with reasonable firewall / passenger diagonal
You can see the flex is reduced by %25 using the "reasonable" solution. That wold be suitable for track use, but no passenger. In this case I run it from the driver side firewall to the midpoint of a crosswise bar under the dash and then to the passenger corner of the main hoop. It is also braced from the mid point of the dash bar to the dash hoop at the passenger floor, This makes a very large and well triangulated section.
The cost for the above change in weight is 7 pounds, 3.2 Kilograms.
The really fun and fascinating part is watching the colors change on the display when these changes are made. It is a considerable amount of additional inforation and ultimately leads to a deeper understanding of the model. Takes some effort though
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We had a very nice picture posted today of a Locost with side pods. I have avoided this because I didn't know how it would work ( look good I suppose I mean ). THe one time I tried this in the model it did not work as well as I hoped, but I think I did not stay with it as long as I should have, It should produce good stiffness numbers. I have been trying to get the stiffness thru tall sides as opposed to wide section sides... Basically just because the diagonal bars above the upper rail are required and I thought they should carry their weight, so to speak.