Probably for a home haunter without access to vacuforming (which would be the great, but really complex to do for multiple props) the easiest/cheapest way to make some snow drift toppers that will form/fit and easily remove would be using carvable foam insulation (the pink or blue stuff). You can cut it, carve it, sand it easily. The trick is going to be fitting it to existing objects and your artistic ability to shape it to look like what you need. LOTS of fine grit sandpaper and making sure you're in a well ventilated workspace (outside is a good idea) and would recommend wearing stuff you don't mind getting COVERED with fine dust and wear a mask to avoid breathing in tiny particles.
Painting it will require layering techniques and brush on latex/acrylic paints (no spray paint - it melts foam). Base coat of medium blue then, light blues, then sponge on some bright white on the upper half, taking care to not cover the blue in the deeper areas.
Mostly you'll need to do a rough cut/glue together, then fit it to each item, remove it to sand and shape, then paint up. As far as attaching, if you got it really snug, then it would likely stay put on its own, but you'll need to be careful with any adhesives since they can pull at the painted layers and potentially damage your existing props that need the snow caps. I really don't have good ideas for attaching so it's removable without damage if you're working with strong winds. Hopefully someone else with better knowledge can help on that part.
Some ideas:
A new addition to my "cabinet of curiosities". It continues the childhood story relic theme I started with Hook's hand from last year. Th...
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^Dave Lowe's bumble tooth is a good example of carving/sanding/painting foamcore with curves/texture that mimics snow. He's added some tartar (the yellowing) buildup, but imagine if it was tinged blue - would look more like the snow drift cartoonish snow you're shooting for.
It was bound to happen eventually. Christmas decorating this year is going over the top like Halloween. Here's a sketch of a fun prop I'm wo...
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The basic construction of the Abominable Snowman footprint for the lawn is really super simple (and as usual cheap). I drew out it's shape o...
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The making of the giant Abominable Snowman footprint holiday lawn decoration continues. When we last left off , the foot print was built ...
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^more of his snow technique using the expanding foam and you can see his suggestions on shaping/painting specifically to get a snow effect. This snow is much more lumpy and that's the main issue I'd be concerned with compared with what you are using as an example. Expanding foam is cuttable/carvable to a point but it is so messy and also tends to become quite brittle after being exposed to air/sun (even if painted). My personal experience is that if it is outside for more than 2 years, it will start falling apart and any props built using it will need repaint/replacing much soon than the sheet foamcore.