A little off topic but close enough I think.
Here's my plan.
I have plants in 3.5 gallon black plastic pots. These pots will fit into 5 gallon buckets. My plan is to put rocks, marbles, hydroton, etc... on the bottom of the buckets at a level where the top of the pot will fit flush. That would leave 6-8 inches from the bottom of the pots to the bottom of the buckets. I would put a piece of PVC down the side of the pots as a fill tube. Think that would work for an on the fly SIP?
It could work. My concern here would be the interface between the hydroton and the soil. I'm assuming the use of some kind of "soil" as a medium.
I'd put some clay balls in the bottom of the 3.5G pot (a few inches deep) and none in the 5G (except what may be needed to keep the 3.5G top flush with the 5G pot - just for ease of access for topdressing - (and some other spacer could work as well)). Then the soil would get watered by wicking from a reservoir filled enough so the water level almost comes up to the interface between hydroton and soil in the 3.5G. I kinda doubt this system would wick well once the water level has fallen below the bottom of the 3.5G pot (the plastic of the 3.5G could be too much of a break in the wicking dynamic). Some folks use fabric pots in a scenario like this as the fabric doesn't break the wicking path (and if this is done, hydroton in the bottom of the fabric pot wouldn't be necessary).
The issue here is that the bigger you make your active reservoir (the bottom part of the 3.5G with hydroton that sits in the water), the less soil will be in the 3.5G.
The PVC pipe wouldn't be absolutely necessary for filling the reservoir (as you could dump water down between the pot and the bucket), but it might be easier/cleaner depending on your overall layout.
A landscaping fabric might serve well as a barrier between the soil and the clay in the 3.5G. Not 100% necessary, I don't think, but it would be cleaner and wouldn't inhibit the wicking.
It might be easier to forego the 5G and just use a deep saucer - one deep enough to submerge most of the hydroton in the 3.5G. If you have tons of 5G buckets, 3.5G pots, and hydroton sitting around and you just want to try some shit, I think this could be made to work, though. An overflow hole drilled in the 5G bucket at a level just below the bottom of the soil in the 3.5G pot would be a safety against overfilling the reservoir and over saturating the soil.
This kind of stuff can be hard to explain in writing, so I'd be happy to clarify if this is a bit opaque.