That's an excellent question! Dealing with a wobbly or tipping trash bin on an uneven patio, sloped driveway, or bumpy lawn is a common frustration. The best base for stability is one that creates a level platform, compensating for the ground's imperfections.
For a permanent and highly effective solution, a custom-built wooden platform is often the top choice. You can construct a simple frame from pressure-treated 2x4 lumber and deck boards, adding adjustable furniture leveling feet to the corners. This allows you to perfectly fine-tune the platform's height on all sides, creating a perfectly level surface for your bin, even on a significant slope. It's sturdy, durable, and keeps the bin elevated from moisture.
If DIY isn't your style, several commercial products work well. Look for heavy-duty plastic or composite deck tiles that interlock. Some brands offer tiles with built-in adjustable pedestals specifically for creating level surfaces on uneven ground. Alternatively, a simple concrete patio slab can work if the unevenness is minor. Just use shims (plastic or composite are best) underneath the slab to level it before placing your bin on top.
For a quick, non-permanent fix, a rubberized or non-slip mat designed for outdoor use can provide extra grip between the bin and the ground, reducing minor shifts. However, for true stability on a noticeable slope, a raised, level-able platform is undoubtedly the most reliable method to prevent spills and keep your bin securely in place.