If it's modest in size, how about EPS insulation (commonly known as styrofoam)?
Light, easy to cut with standard tools, easy to carve/sculpt, sands well, "never" rots. To paint, coat with Kilz, sand a little then use any latex paint.
You can get the exact color you want (the Grinch is VERY particular) at big box then buy a sample size for about $7.
The biggest downside is ESC (epidermal static cling). No drug for it, but a good shop vac does wonders
a large amount of my halloween stuff is foam based,, holds up well, easy to store,, I try to design things so they pack-up flat,, this years possible adds,, I ate all that kale for nothing!! and Tim Burr, very bad name for a lumberjack!!
I used to make a lot of yard art. I buy my patterns from The Winfield Collection. We were looking at a shed to put them in! Great memories!
Marine plywood or outdoor sign material.