Is the Top Hat better than the standard Thien seperator?

[retired2]

I suppose something could also be 3D printed.

Hi Phil... Newbie here. 3D print is very doable if the printer's print volume is large enough. What would be the overall dimensions of the ideal "round-to-square" inlet for a 30 gal or 55 gal drum?

"Ideal" is round to rectangular, where the rectangular area is about 10% greater than the round area.  My 5" round to rectangular transition is about 9" long with a few inches of neck on each end.  The total length is about 14".  The length of the transition was determined by the sheet metal fabricator.

Search "rectangular inlet" on this forum you will find many useful discussions.