After much trial, I figure that round tubes that I use for plants support is of right diameter to get the correct height and if I lay them side by side in the gutter they will make a raised platform for pots to sit.
It work ! Five tubes can be place in the gutter making a raised platform and the pots is stable on it. Since it's a hollow tube, it doesn't displace much water volume and light in weight. I could drill holes making it even lighter and helps water circulation at the same time.
These tube were from my staking sets, I'll need to buy new ones and cut to correct length.