I made about the same thing. And when I used it on pavement, would put a cinderblock on the back of it. Hid the cinderblock in the bushes so it was handy. And took off from the outer most of the the circle. When landing, I'd make sure I was center circle away from the side the block was on.

When I moved to another state and got permission to use an unused ramp area at an airport, I searched the concrete until I found a seam that had a couple of holes in the fill asphalt (asphalt seam filler in the concrete). I drilled a couple of holes in my stooge that suited the holes in the seam and was good to go without needing a weight.

Modelers can fix anything.