Apologies in advance for my ramblings, I've been pondering....
Initially I was thinking of ways to mount the water inlet on the lower part of the bucket to force the most water into the fluid bed scoop while the rest would flow up and around eventually down the sluice. But you could possibly also run PVC down to the bottom and then have an adjustment to let water out for over the slucie.
Any way, mind was wandering to the fit around the sluice. Wedged shaped so wider at one end over the other means if you cut the exactly needed size, the sluice has to go in through the bucket. Possible work around there is a shorter bucket that would allow you to angle the sluice in.
But then I had also seen the first video below and started thinking about shoveling dirt straight into the bucket and how any material not heading out the sluice would or could end up in the fluid bed intake and plugging them holes isn't an option.
So then I started thinking about a funnel, something to direct the dirt directly onto the top of the sluice and out towards the opening in the bucket. Well, naturally another bucket with a notch to direct the material onto the sluice.
Well now, this is where the brain took a left turn and anyone familiar with C-clips or brake hose retainers may see where it went.
If you were to cut the base out of a second bucket and cut a slice the width of the sluice where it enters the first bucket, as you slide the second bucket into the first, the second bucket will keep the sluice from sliding out.
Then, cut and bend the other 3 sides so they direct the material onto the sluice. The water can be pumped into the outer bucket towards the fluid bed intake, and as it fill the void between buckets, flow up and out slots cut towards the top of the second bucket creating the flow over the top of the sluice.
Some others using similar buckets