I’m always a little too excited about projects that turn odd household leftovers into something genuinely useful, and this one checks every box for me. These DIY heavy-duty deck post caps use melted plastic ice cube trays plus two basic pantry staples to create a sturdy, weather-tolerant cap that helps protect the tops of wood posts from standing water.
If you’ve got aging deck posts, fence posts, or even garden structure posts that need a simple protective top, this is a practical weekend project. I like this kind of make because it feels resourceful without being fussy, and once you get the process down, you can batch out several caps in one session.
Materials
Instructions
1. Measure the top of your deck post so you know the finished cap size you need; most deck posts are 4 x 4 inches or 6 x 6 inches, so plan to form each cap slightly larger for a snug fit.
2. Wash and dry the plastic ice cube trays thoroughly, then cut or break them into smaller pieces about 1 to 2 inches wide so they melt more evenly.
3. Pour the salt and baking soda into separate small bowls so they’re ready to use; these help keep the softened plastic from sticking while you shape it.
4. Set a sheet of parchment paper on your work surface and lightly dust the center area with a thin layer of salt mixed with a little baking soda.
5. Melt the plastic tray pieces using your preferred controlled heating method until they become soft and moldable, then gather the softened plastic into one mass on the prepared parchment.
6. Sprinkle a little more salt and baking soda over the top, then press the plastic flat into a square or rectangle slightly larger than your post top and about 1/4 to 3/8 inch thick.
7. Fold or press the edges downward to start forming a shallow cap shape with a flat top and short sides that will sit over the wood post.
8. Test the fit against the top of the post or a scrap block cut to the same size, adjusting the sides while the plastic is still warm so the cap slides on without cracking.
9. Let the cap cool completely until rigid, then brush off any excess salt or baking soda from the surface and edges.
10. Repeat the process for additional post caps, using fresh parchment and small extra dustings of salt and baking soda as needed for easier shaping.
Variations & Tips
For a cleaner fit: Make a simple wood form from scrap lumber cut to the exact size of your post top. I do this whenever I want multiple caps to match, and it makes shaping much faster.
For drainage: Shape the top with a very slight dome so rainwater runs off instead of pooling. Even a subtle raised center helps a lot outdoors.
For thicker caps: Use more melted tray plastic per cap and keep the top closer to 3/8 inch thick. That extra thickness is especially useful on posts that get strong sun or rough weather.
For smoother edges: While the plastic is still warm, press the perimeter gently with parchment paper between your hand and the cap. It’s a small step, but it gives the finished piece a more polished look.
Batch-making tip: I like to measure and prep every post first, then make all the caps in one round. It’s the same reason I meal prep on Sundays: a little organization up front saves so much time later.
Important safety note: Only melt plastic with a safe, controlled method and good ventilation, and avoid overheating. If the plastic scorches or produces strong fumes, stop right away and discard that batch.