got some thin plastic from local model shop that could be cut with a sharp knife. I then cut sort of U shapes and layered them up until they were tall enough and secured them in place either side with longer screws through the original battery posts (no need for extra holes)
I no longer have the car sadly so can't take any pictures
Alternatively if you are using stick lipos you can take the original stick pack holders and sand them flat on the top (assuming you have square lipos) and then just use the standard cable ties to hold it in place - if using rounded lipos they fit straigt in.