A $20 four foot level will work just fine. That and a STRAIGHT 8' 2x4...

Uhhh... Noooo...

I was mainly using that as a comparison. I like the idea of making one with a hose if you can find the clear hose to make the viewing area for each end.

Yes, and a chalk line. Mark one point on the house, then another point on the house 8ft away with your level and 2X4, then snap a chalk line through the two points past the 15 ft mark.
To the OP

Set the tops of the 2X4 at the chalk line and use the level to set the 2X4s. If you want a slope use a piece of 1X (3/4 inch) n the "low" end of the level and you'll have 1-1/2 in slope on 8ft.

I used a water level (clear tubing) on the last house footer I did, it works fine but you don't need that for what you're doing.

BTW present day 2*4s 1-1/2 X 3-1/2 inches.

"BTW present day 2*4s 1-1/2 X 3-1/2 inches."....

I knew that, but why I wrote 3 3/4", well.....umm.....
It's to dang hot(embarrased grin).

Have fun...notnew...

"too" dang hot. (before CC gets you, LOL)

That's "Before CC gets you, LOL."

heh heh heh