You can do a modified Durso pipe by using, for example, 1.5" PVC for the standpipe, and then topping it with a 4" slip endcap. The key is to have something holding the endcap above the top of the standpipe, so that water can flow down the pipe. The lower rim of the endcap should be below the water level in the overflow chamber.
My preferred method is to just cut the top of the standpipe to leave two or three very thin "extensions" that will hold the endcap. This allows water to flow under the cap and into the standpipe. Then just drill a hole in the top of the endcap (or place a valve there or do one of the other variations for allowing air in).
I hope this is clear. Just keep in mind the principles of the Durso standpipe. It's just having the water input below the water level, coupled with an air input to break siphon. That's it. The original Durso design and this end-cap design are just variations on the same theme.
