The RM cage is a short cylinder made of perforated steel with formed ends turned in as flanges to positively clamp the filter.
I was lucky enough to pick up a NOS cage for ten buck at Broadford Bonanza but was going to make a cage for a spare change over filter.
I reckon you could make a passable item from some mouse mesh netting - 6 mm squares- with some soft tube to make the ends.
The other way would be to have the perf laser cut leaving a strip of plain at each end to form the slight shape needed. Perf dia. pitch pattern is 8mm on 10 staggered. Gauge is .5 mm.
The filters are available from Suzuki and they may even have the cages.