There’s no need to use expensive, cabinet-grade plywood that’s perfectly flat and voidless for a mini-golf hole. It’s going to live outside, you’re going to sand and paint it, and it’ll be trod on, kicked, bashed with golf clubs, and otherwise abused for months. Plywood sheathing will do just fine. It does come with some tradeoffs, though: it’s full of voids (gaps between the layers of wood), it’s inconsistent in thickness, and it’s absolutely crammed with glue between layers to make up for the lower quality wood.

Attach it to the spoilboard as flat as you possibly can, and use more screws than you would with high-quality wood. As you cut pieces out of the wood, the wood will likely deform even more as you change the internal stresses. It’s helpful to counteract this by adding more screws.

A quarter-inch down- or up-cut end mill will work just fine. Don’t use anything smaller - the large amount of glue inside the wood will wreak havoc on smaller bits. Design your artwork to avoid any curves that your bit won’t be able to reach.

The voids inside the wood mean it’s possible that a retaining tab will end up missing because there isn’t any wood there to support it. Add extra tabs! Plus, the wood will likely warp as you remove parts of it, so ensure each curve in your design is well supported.

If your plans don’t involve tool changes, consider using the spoilboard as your zero-reference, so that the inconsistency of thickness over each sheet of plywood won’t matter as much: you’ll always be guaranteed to go through the material eventually. Similarly, measure each piece of plywood separately and re-calibrate your toolpaths to suit: There can be as much as a quarter-inch of difference in thickness between individual plywood sheets. If your toolpaths do involve tool-changes and you don’t have access to the spoilboard while the stock material is on there because it covers the whole thing, you’ll be stuck with using the top of the material as the zero reference. This is where trying to keep the material as flat as you can is of critical importance: try and make the material as flat as possible with extra screws (see above) and then take your zero-reference from near where the next cut is going to take place.