Instinctive answer

Dear Sancho,

Depending on the intended use of the diorama,
(is it intended to be standalone, or will it be integrated into a layout at some future stage?)

and the required longevity of the diorama,

I would suggest:

- Matteboard (basically thick artist cardboard, dimensionally stable, takes paint well, nice surface finish. quick and good-looking result for short-term mission)

or

- Foamcore (3/8" thick, easily cut and shaped, provides a decent surface for paint/texture/carpet, surprisingly long-lasting for touring and "hand around at convention clinic" demo pieces)

You could try "skreed-ing" on anything like plaster (fragile, doesn't take impacts at all),
Acrylic Caulk (hard to get a nice consistent smooth surface, some paints may not stick),
or similar, but in this position, Foamcore would be my choice...

Happy Modelling,
Aim to Improve,
Prof KLyzlr

Acrylic Spackle May Work

Acrylic spackle may give you a tougher finish than plaster, yet capable of being sanded very smooth and will accept almost any paint. I haven't tried this, so test it out first!

Wallpaper ?

Wallpaper ?

Lightweight sparkle (sold by

Lightweight sparkle (sold by Woodland Scenics as “foam putty”) will do the job. Apply with a pallet or small putty knife..it’s sandable, but you can get it very smooth when applying. 

My experimental solution...

Thanks for the recommendations everyone!

I ended up trying something that I had on hand and it seems to have worked out fine. My intent is to add the diorama to a layout module in the future but nice enough to hang out on a shelf in the meantime.

DAP DryDex is similar to a lightweight spackle but is more gelatinous in consistency and has more grip to other construction materials such as foam. Two applications and some sanding have resulted in a near perfect finish. It isn't super durable but plenty strong enough for gentle handling.

I'd love to see some more experimentation in this area!  

Have done

Dear Sean,

yep, have done, which is why I reccomended Foamcore...

Happy Modelling,
Aim to Improve,
Prof Klyzlr