All of the strong and flexible products are nylon based, and nearly impossible to sand and is very rough and dimensionally less stable for holes and such.
Anything that is requires a good finish I would choose FXD. For underbody stuff FUD is fine.
Just remember this. The SF materials require much thicker walls and that translates into more expense for FUD and FXD that do not require it.
If I were designing this I would have designed them for FXD and not even offered SF materials.
If you look on my site, there are some pics of BSF parts that I use to hold DPDT switches to Atlas turnouts. There are some links there to instructions with more pics.www.keystonedetails.com