Personally I would just go without corner flags. All well and good improvising, until the ball hits the "flag" at force, the umbrella deploys and affects the path of the ball, or even worse a player falls onto it and hurts himself.
There's another topic on in the spirit of the game, and for me missing corner flags fall into this category. Yes, the laws say they must be used, but any referee refusing to play a game because they were missing would not, in my opinion, be acting in the spirit of the game. After all, games have been played on hundreds of astro-turf pitches over the years without corner flags.