I just installed my Uni Filter air filter in my 2020 ZForce 800 Trail and the fitment is perfect.  I know others have used p/n 6229AST and have had to trim the intake tube to fit this filter.  The 8703ST fits the intake tube perfectly.  The filter has an approximate 5/8" thick flange and the outer bead on the intake tube slips all the way into the filter flange retaining it from the inside while the inner bead sits just outside the filter flange allowing the clamp to securely hold the filter between the two intake tube beads.  In addition to the Uni Filter I also added a K&N 22-8016 Precharger filter for a bit of additional protection.

[2015 Z Force 800 53/Trail EPS]

Great. And what filter oil are you using on your Unifilter?

I'm using the spray can oil that comes in the Uni Filter cleaning and oil kit.