Filler putty - building panels

[Chelsea]

Hi just a question what's a good putty or something to build a new panel ?

I'm trying to build a new panel section to conceal the undercarriage (making undercarriage doors)

Thanks everyone

[ElCapitan]

Does the kit not come with any doors?

[Chelsea]

Hi it came with door that sit open, so I modified them and airfix them in a closed position now I need to fill and sand it to completely finish the job.

[digger303]

a picture would help tremendously