The plumbing test caps are the best plug to use hands down. For years we used the threaded PVC caps and it was a real pain. The test caps go right in and out and don't let any water by.
The test caps work great if you can match up size. My case I need 3 1/2 and they are either 3 or 4 inch caps. Those test plus fit inside pipe diameter, the threaded fittings the inside diameter are 1/2" larger.
You need to change out the center bolt & wing nut to galvi or stainless also. Keeping the bolt lubed if a good idea also.
I keep my PVC plugs lubed with some grease on the treads and they work just as good.
The best solution is to raise the deck & scupper height which I've been considering doing to mine for a few years.