Many thanks for a positive plethora of ideas!

Oddly enough I saw a video about super glue and baking soda shortly after posting the query. Looks interesting
Fibreglass resin and Isopon I have used it in the past. It worked well for the purpose but I always seemed to get more on me so I am a bit wary. :-(
“Green Stuff” does sound familiar, thevtube I had seemed tindry out too wuickly and became granular, not good for sanding. Perhaps my tube was old stock.
However, I will look out for some of the products suggested..
Thanks again.
These are what I am filling.
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