You could simply take a hardware store tarp and drape it over the boom..lightweight and stows easily.
If you want the 'arched' shape, you can carry some lengths of tubing/plastic pipe and drape the tarp over that.(and/or sew tubes into tarp for the pipes) This would stow in a long, narrow package when rolled up and may be doable if you have somewhere to put it.
A pack of bungee cords will complete the package.
As a temporary thing on a daysailer I don't think you need to overthink this.
If you want the 'arched' shape, you can carry some lengths of tubing/plastic pipe and drape the tarp over that.(and/or sew tubes into tarp for the pipes) This would stow in a long, narrow package when rolled up and may be doable if you have somewhere to put it.
A pack of bungee cords will complete the package.
As a temporary thing on a daysailer I don't think you need to overthink this.