how about tofu? you can buy a package of extra firm tofu, slice it to about 3/4" thick. In order for it not to be soggy, you need to press it. So take the 3/4" slices and put them on some paper towels, put some paper towels on top of them and then place a heavy object on top (usually I put a plate or cookie sheet on top and then a heavy cookbook on top of the plate). Press it for at least 30 mins. You can add it to the rest of the spring roll ingredients just like that or you can season it with some thai seasoning and then put it in the roll.
Another option is tempeh. Buy a package of tempeh, chop it up into cubes and fry / sautee it for a few minutes until it's crispy. Add that to the rest of the spring roll ingredients.
hope this helps