Set specific guidelines for student projects. It's important for students to know exactly what a model should look like and do before they begin creating the model. If you're looking to find a model for students to base their version off of, you can refer to an exemplar from a previous year or order one from a catalog. Sometimes, it's helpful to create a sample model in front of the students so that they can see the process of building a model and then have a model to refer to as they construct their own.
Attend regional science fairs and art competitions. Many localities have regional competitions where students demonstrate models that they've created for the local community and other students. Attending one of these fairs or competitions will give you the opportunity to see what others are doing and to recreate some of the ideas within your classroom. Many students thrive on competition, so sometimes, attending a model fair can give students the idea to improve upon a previous idea and make it their own or make it even better.
Purchase a science or art model kit from a catalog. There are catalogs that offer science experiment kits. The end result of performing the experiment is a model volcano, DNA helix structure, brain diorama or the like. Art model catalogs often demonstrate some specific principle of sculpting, jewelry making, etc., with the end product as a finished model. Keep the best models for future students to aspire to and exceed so that there's some demonstration of what students are capable of. Teacher created models don't have the same impact, nor do they inspire students to achieve higher than those who have come before them.