MP Select Mini - Spindle Rounder with stop

A remix of the spindle rounder, which helps prevent rattling of the spool. This remix includes a stop to prevent the spool sliding down and dragging against the printer. I've also made the spacer long enough to prevent the spool dragging against the [SD Card Extender Holder ](https://www.thingiverse.com/thing:2311469), if used.

N.B. unlike the [Remix MP Select Mini Spindle Rounder with Spacer](https://www.thingiverse.com/thing:2892443), this does not have a [split in the model](https://www.thingiverse.com/thing:2892443/comments/#comment-2174604), so it prints perfectly fine.