Anyone know where I can get a reasonably priced waterfall
weir/spillway? You know, that box the water comes out of at the top of
the falls.

Everything I see is on the order of $75 new and $50 on ebay. That's way
too much for hunk of plastic in a handy shape. And the whole "it's a
filter too" thing is silly. It's still a hunk of plastic, even if it
does have a sponge in it.

Has anyone built one of their own? I was thinking that you could put a
bulkhead fitting onto some container of a similar shape. It seems to me
that the most important thing is the lip at water exit, so that you can
tuck the stream/falls liner under it.

A cut down plastic storage container perhaps.