Assuming you're going to paint the finished result.........
Having had a couple of not very flat and (and sometimes knotty) pine window
sills from my local timber merchant which I still ended up cutting to
correct depth, I've just done one with an offcut of 16mm plywood sheet from
when I floored my bathroom - with the front edge faced with pine decorative
strip. Result is dead flat and looks good.
Cutting your own from sheet might sound like more work but if your required
depth doesn't match whats available, it isn't really. Also suits odd shapes
like bows too. Most timber merchants will cut to size for you for an extra
Paint well though (I sealed the hidden edges before fitting too) - standard
plywood doesn't like getting wet if you have condensation problems. You can
get waterproof "marine" plywood but its a lot more expensive.
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