There is another way to do this. especially if you need it to support models ie. Weight.
Use thin guage wire, penetrated through the base. The first layer should be a smallish piece of cork-board (cork board tiles) with the wire pushed through it and then pushed into styrofoam, then cut to whatever shape you need (small spires) in this particular example.
The reason for the corkboard is that ive found just pushing wire through a models plastic base doesnt stiffen the wire enough and that styrofoam isnt very strong. BTW Styrofoam when using PVA glue, bonds better to corkboard than plastic - dont really know why it just does.
the other option is layering corkboard pieces one on top of the other - this is what i do and then just hack the pieces into whatever shape i need. It doesnt look very smooth (smoothed out by wind erosion for example) it generally looks quite jagged.
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