Each slit would be 3/4'' to 1'' in width and they'd be spaced about 1-2
inches apart.

Any fish that might find their way through openings that large? A nice long
pipe with slits like that would be a very tempting refuge for smaller fish.

Another thought: I'm no fluid engineer but I suspect 99% of the water going
into the drain would enter through the slit closest to the closed pipe that
takes the water away. You'll probably get no draw from the end of a 5' length
of pipe.

If you go ahead with this design, I strongly suggest you dig the pond so there
is a channel for the pipe to lie in, have the slits on the underside, and slope
the floor of the pond so all the gunk gravitates toward the pipe. Keep in mind
this would be difficult to clean should the slits become clogged with leaves or
some other stuff.

I'm very happy with my dome drain.

~ Gary