Galvanized horse or cattle troughs make cool looking and functional planters but I have never seen them converted for sub-irrigation (aka erroneously as "self-watering") until now.
The website Mary Janes Farm presents a very well done PDF instruction using sections of PVC pipe to support a perforated soil platform. Variations of this method are in widespread use, particularly for making tote box SIPs.
If I were making a trough SIP, I would use this simple method that does not require power tools or PVC pipe. The same method was used to make recycled 55 gallon drum planters called GrowBarrels in Minneapolis.
My highest level of frustration continues to be the fact that we do not have a consumer demonstration garden here in New York. If we did, you can be sure there would be a galvanized trough SIP on display along with a 55-gallon drum SIP, a traditional wooden raised bed SIP and many others.
This city prides itself on progressiveness yet sadly lives in some prior century of dirt-centric horticulture. We can thank a relatively small group of reactionary obstructionists for that.
See prior posts about galvanized cattle or horse trough planters (none of them sub-irrigated).