Spackle I don't think would be a particularly durable way of getting this done, but I do have an alternative.
For my Shanks gun I wound up having to coat foam with bondo, then put fiberglass cloth + resin then more bondo. This gave a VERY strong outer coat, however it was labor intensive. I have since revised this method to just be masking tape, fibgerglass cloth + resin and bondo. The reason for the initial bondo/masking tape layer was to prevent the melting of the foam. I detail the build on my blog!
Good luck to you!!!