I don't know about you, but I use a lot of spackling-type products when I'm basing up figures or scenics. I use it to smooth out the transition from the figure's base to the base it's being mounted on.
Above you can see a container of spackling that's available at Dollar Tree. I was skeptical about the quality, but I was low on spackling, so…
For wargaming purposes, you'll also need something smaller than the usual tool for applying spackling to walls. Fortunately, Dollar Tree sells a set of those, too!
To my surprise, this products seems to be functionally identical to 3M Wall Repair – a powdery substance that turns into filler as you compress it with your contouring tool. I've been working with it for a few weeks now, and I find that it's quite useful.
It supposedly dries in 20-30 minutes, but I find it needs a few hours (depending on how thick you apply it). When it is completely dry, you can't make any imprint in it. If it is not completely dry, and you try to paint it, you'll get some of the spackling mixing into your paint!
When it is dry, you can shape it with the contouring tool, a dull knife, a sanding block, etc. Note that the longer it dries, the harder it gets – if you wait a few days, you'll find it's become quite tough!
It cleans up with water, and I've noticed no shrinkage issues.
Note that according to the packaging, this material is not for use on plastic or metal. No explanation given. I've used it on plastic and painted metal with no problems.
It's also handy around the house!