Hi,
The trick I used to fit thicker packs was to replace the long setscrews with actual longer screws.
After tightening the screws all the way in you have to cut the heads, keeping attention hot to generate too much heat.
Having the strap fixed just by the nuts is not going to be an issue, if you can avoid big crashes.
For added security I think you can build something to fasten with the rear tower screws where the strap can lean against.
B4 was and still is a great car!
