An aging round soaker hose sprang a leak large enough to gouge a crater under a tomato plant, so I conjured a short clamp from the longer round hose splints:
The shiny stuff is the plastic backing on strips of silicone tape intended to prevent the high-pressure water from squirting through the porous 3D printed plastic. The fat drop hanging from the hose shows some leakage around the tape; an occasional drop is perfectly OK.
The leak faces the round side of the bottom half of the clamp, which probably doesn’t make any difference.
I hope the washers occupy enough of the minimal surface to render aluminum backing plates superfluous:
Creating the 3D model required nothing more than shortening the original splint to 30 mm with two screws along each side. While I was at it, I had Slic3r make three clamps to put two in the Garden Dedicated Hydraulic Repair Kit for later use:
Change two lines in the OpenSCAD code and it’s done.
Also: clamps for flat soaker hoses.