Large battery hold down
 

I have a Redcat Lightning EPX and a 36.75mm tall 3S SMC LIPO. What type of battery hold downs do you guys use for this type of setup?

I used the stock battery bar...but I bought a set of body posts (which are pretty tall) and cut them to the height needed and put them in place of the stock battery posts. Nice and simple.
Yours may still have the little rounded bumps on the bottom of the battery area...these are there to hold NIMH packs in place but don't fit most lipo packs. 1 screw under the chassis holds each one in place. Remove them and that will let the battery sit a few MM lower.

If you remove those bumps, the lipo may be able to slide a bit even with the battery bar snug. To keep that from happening, I put Velcro on my lipo and on the battery bar.