FWIW, attached is how I modified my HF DC. Very similar to yours except the Dust Deputy mounting flange and my exhaust goes through the canister filter before dropping in the bucket. My plywood sits atop the trash barrel without any clamping and has a groove milled that fits the rim with foam insulation for a seal. I did not modify the stock impeller. Big qualitative (I didn't measure) improvement in flow over stock HF.
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