I've been down this road a number of times. Never quite got to building my own wheels, mostly because you can't find suitable tires for less than $130 each. I just forked over my >$500 for a new set of Cat Trax, simply because they are the only wheels I've used that work well each and every time, no matter the surface. Other beach wheels don't roll easily on soft sand, weigh more, and are easier to puncture.

Your problem however is one that I sympathize with. My boats live on the beach during the summer, which is a 5 minute walk from my house. In general, I'm not worried about stuff getting stolen, but it has happened, and given the current economy is more likely than in prevalent times. I've also had a set of beach wheels stolen or trashed at a different location (not a beach or waterfront for that matter). I doubt these new wheels will be spending time overnight on my beach. So I am relegated to carrying them back and forth, or moving them on the trailer, all of which is a huge pain I'd rather not deal with. The other solution is to lock them up, which I might do on a weekend. As to solving the alternative problem, PVC pipes work pretty well as surfaces to slide the boat on and are very inexpensive compared with wheels.