is there a example pic of what you want ? , maybe that would help
... what i understood was ... you want to put a ITX on 4 5.25 bays ? , as i remember , a ITX is 6.7 inch x 6.7inch
its not gonna go in horizontal position , its going vertical , i can give you the hole patern for the ITX motherboard tray , you just gotta print it out , check if its the real world measurements ( just in case the printer scaled something lol ) and do some drills and taps for the tray ...
then , after you finish the motherboard tray with some standoffs and all , now you need to drill some 5.25 holes on the ITX mothebroard tray , make some taps and there u go . a ITX motherboard tray attached to the 5.25 wall
the fan part , i assume it would be easy too
just need some plexi cement glue thing and the 120mm fan hole pattern with the holes , and again some tapping , and done
... i could fire up solidworks and see if i could help u
but it would be great if you had something more visual , a drawing or something , but i cant actually make the "part"