Your avatar photo shows what appears to be OEM side bags. Is that the case (no pun intended)? That might matter when it comes to suggesting top case mounting methods.
The cargo carrier you see in place of the rear seat is most likely the one made by HONDABIKEPRO. Is is very well made but would not fit your requirement of being inexpensive.
Just Google "showkey mod" for a suggestion on how to remove the forward slope of the stock seat at minimal cost.
Is your old trunk a popular brand that might have specific mounts available?