This how I did mine, I was able to use (2) 3s 5400mha lipos in the 3905. I think the straps cost me $15.00?? it’s easy / no issues at all.. PS. use the Traxxas 6 cell foam to fill in the gap so that the lipos don’t slid left and right.. (see first picture)
Hope that helps??
Have a great one..