KZ400S Ducts, Air Cleaner

Does anybody have a workaround or suitable substitute for part 11015-052, Duct, Air Cleaner for a 1976 KZ400? It's essentially just a rubber tube 53mmOD x 47mm ID x 3mm thick, and the ones online are insanely priced.

With a little google search: www.cruzinimage.net offers a set of 2 duct boots for $26.95

www.cruzinimage.net/2018/11/15/76-77-kaw...-box-duct-boots-set/

I have seen people use pieces of radiator hose for that; it seems to work pretty well, though it may take some time to find the ID and OD.

Yeah, I think I'm going to go the radiator hose route - the air cleaner duct is 57mm OD, 47mm ID and 3mm wall; I found that a hose/tube that measures 1 7/8" ID is actually 48mm - so that works.  Along these lines, I'm looking around for some carburetor passage plugs for my '76 KZ400S model...this post here  Rejetting CV36 Keihin carbs for a KZ400 B2 engine - KZRider Forum - KZRider, KZ, Z1 & Z Motorcycle Enthusiast's Forum   lists some Suzuki plugs for a good price rather than the $25 apiece listings I've seen elsewhere - but will they fit?? I assume so, or I wouldn't think it would be in the post - right? 

I'm also looking for two PLATE, MAIN JET - #16055-007 for the carbs, and I think - but I'm not sure yet - that the CV carbs that are currently on the bike might also be missing the springs that go under the cap (??).  I'll have to go take a look at the carb body and get the proper number off it to be sure. The Kawasaki manual and website does not show a spring in the diagram, but what else would make the needle go up and down - just throttle pressure?  Anybody have those parts that they don't need...? 
Here's the bike itself:
 

There are no return springs under the caps. This kind of carbs have full metal piston style sliders instead of the rubber diaphram type.
The weight of the slider body itself do the return motion.

Okay, good to know; I didn't see one in the diagram, but it's good to know there isn't one. But the carb (when I opened it up) did not have the little rubber carburetor passage plugs or the Main Jet Plates either; do you know if it still needs these? And will those Suzuki plugs I listed earlier work?