How big is too big for a RMH riser?

Howdy, I am assisting with an old school build in Europe, just as a labor of love.  My friend is following a video by Uncle Cy.  Barrel is cleaned up but not cut, and I see that he has soooo much insulation around the riser,,,,,it will be a tight fit to slide his barrel over the riser.

The cob he layered has plenty of straw and Nexclay, a thermal insulation made from clay /perlite.  It is still moist and pliable.

Any and all opinions will be met with glad tidings and joy, thanks