The problem with normal concrete is that it's quite heavy, and that's no good for something that needs to be portable. There is some kind of special low density concrete that could work, but it's quite expensive and prone to cracking. The bed liner is a good idea, but it seems like it would be a really high traction material. I have an extra trailer, and I'm thinking about building a rock course on it and taking it with 5 crawlers to places like the company picnics, family reunions, maybe some birthday parties for family members (who are old enough to handle my crawlers), things like that. I think it would be a great idea to allow people to see RC crawling up close and personal, and a good reason to get one more crawler, since I only have 4 now, and more is always better lol.