The simple method is to use a standing train as a big box. It has to include a locomotive and needs to be parked at a station, though, just so you can read the train contents. Up to six kovarex machines can work on the contents a wagon, and you still have to spots at the rear where you can place inserters to refill / skimm off. As usual with simple setups, you won't get anything out of it until all Kovarex machines have filled their buffer.
Another variant to the sushi belts above:
U235 is fed back into the machine just as it comes out. With hand_size = 2, that takes exactly ten swings until it has enough to start the next batch, at which point it demands U238 -- this takes another three swings, while the excess item of U-235 happily slides past. So even without any circuit magic, I can rake off some U-235 right from the start.
However, without circuit magic I cannot ever be 100% perfectly positively sure that it will always work like that; so I'm careful to only take what I really need while the remainder goes into the machine's buffer. And anyway, that scheme will no longer work as soon as I add a second centrifuge. A this point, i install a throughput limiter for U-235. One item/tile is allowed back into the loop, anything that piles up at that chokepoint is siphoned off.