different type of slingshot

Hello all...I have built a giant slingshot to shoot bait from the beach. Rough size is arms are 9 foot tall and about 4 foot apart. I will be shooting 4 to 6 ounce sinkers and baitfish..so maybe a pound or two total weight. For practice i bought a water balloon launcher from amazon and it does ok(i launched some decent sized rocks with it) but i need more distance. Should I go with the flat workout resistance bands? Round resistance bands? Heavy weight or lightweight? Tubing? Any suggestions would be appreciated..I have never done this before so I am lost..I would like to get the bait out as far as possible so I don't have to wade out into the dark ocean at night..cause it can be real creepy..LOL Thanks again for any ideas!!!!