Bud, don't sell yourself short. There are plenty of things you can do to create some custom Tyranid objective markers.
Tyranid Objective Marker Ideas.
1. Tyranid Infestation Markers: Go grab a box or two of Genestealers. They keep several infestation markers in each box. Slap it on a base and you are good.
2. Tyranid Egg Clusters. You like nuts? Regardless, pick yourself up a small bag of pistachios and some green stuff. Maybe some liquid green stuff if you want to do that route. Now, follow the following instructions, and profit.
Instructions:
2a. Eat the dang nut, and keep the shell.
2b. Use the green stuff on the edges of the shell to keep them together, with maybe a little liquid green to smooth it all over so it is one seamless egg shape.
2c. Paint it to style and plop it on a base.
2d. Profit.
3. Hormagaunt Egg Cluster: It's even easier than the last one. You just need green stuff.
Instructions:
3a. Take green stuff and make your snake. Make a ring along the edge of the base.
3b. Smooth out the edge to simulate a dug pit impression. Then proceed to make a second snake.
3c. Remove small pieces of the snake and shape into small egg shapes. Use a Hormagaunt miniature to reference size and make it seem like it could actually squeeze out of it's nether regions.
3d. Place the ggs in the nest, paint it and admire the fact that you actually paid attention to Tyranid fluff and made something fairly accurate and is now home to close to a dozen unborn super predators that will soon hatch and rip some hapless guardsman's face off. d'awww