It is an easy thing to do. It won’t be a balanced connection. The RCA does not support the return wire (pin 3 of xlr). It must be left lifted at the RCA end.
I have jensen transformers on one of my amps. they are fully balanced. the box in the link above appears to be an adapter box that can be just as easily done with a cable. Maybe they do something else inside that box but I can't tell by looking at it.
I built a pair of RCA to XLR connectors to run my HT bypass from the RCA out of my HT preamp to the XLR inlet of my amp.
Jerry