How do you diagram a sentence that has a nominal phrase that is described by an adjective using the Reed-Kellogg system?

I hope everyone had a nice holiday weekend.

"Holiday weekend" is not a compound noun, but a syntactic construction: a nominal with "weekend" as head and the noun "holiday" as modifier. "Nice" then modifies the nominal "holiday weekend" to give the meaning "holiday that is nice by the standards normally expected of holiday weekends".

Incidentally, the Reed-Kellogg system of diagramming has been intellectually bankrupt for over 100 years, and (to the best of my knowledge) has never been used in the UK. The diagram fails to show the syntactic structure and the function of each constituent.

If you really must use the Reed-Kellogg diagram, the best you can do is add the modifier "holiday" to the same line as "nice".

Edit: Below is more conventional tree diagram, from which it is clear that "nice" modifies the nominal "holiday weekend".