It came to me in a dream, the snail.
Floating down from the sky on turps fumes, it spoke: (In the voice of Michael Clarke Duncan).
"Fear not the silicone, young grasshopper" It boomed. "Calm yo'self. Study me, and follow my technique, then build. Build!." It said as it slowly turned and snailed away, leaving a nice, smooth, even line of snail goo.
So I did another one. This time it went a hell of a lot better. For sanding I took an old table frame and cable-tied the drill to it with one of those sanding disks. Sanding by hand took 3 smoke-breaks. This was way, way quicker.
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I followed Cascade's advice, but couldn't find a putty knife, so I took some arb card thing that came in the mail and used that instead. I used the round corners, then cut the edges with the corner I angled. The gap on the side is for the hinge.
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The result:
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