how do you keep sluice sections from leaking
That doesn't bother me when I see it -- however -- if it seems to be a source for loss for you just put a catch pan under it and pan it out to see if you're loosing much or even any. If it does seem to be a problem then thin, flat, weatherstripping foam with pressure sensitive adhesive on the inside box bottom overlap would probably seal it up enough, Won't take much if it does need it. be sure it is dry and very clean before you try that (if needed).
I try to remember that I'm just not going to catch every minute bit - and - if I just shovel two more minutes at the end of the day I will very probably cover any piddly losses I might have sustained all day long.
Most of the time the leaks seal up with mud within a few minutes. If you find they don't you can put a small strip of moss between them when you join them. We have tested the "leaks" and found that gold does not travel backwards enough to exit. That is what the water is doing.