ground flax seed should help

Add a little corn starch or, wheat gluten to them. Wheat gluten is what the commercial burger makers seem to use.

You can put a slice or two or American cheese, a lot of bacon bits, and a boiled egg in the burger. Yum, yum.

You need a starch or a protein. If you are serving the veggie burgers to vegans then that eliminates eggs. You can mash up garbonzo beans (chick peas) for example. I would suggest peel and roast some yams. Mash them up and put them in your veggie burger recipe. That would taste terrific!

all you need to do is add a raw egg, blended, for each 3 burgers.