Improved Main Tensioner (Remix) for TEVO Black Widow

Remix of the most excellent y-belt tensioner by MODesign
https://www.thingiverse.com/thing:2265301
I ran a print which extends off bed at a height, but the bed cradle hit the nut for the tensioner, so I've made a small tweak so that the nut can sit lower and give me more y-axis movement.
V2 update - took a smidgen off the sides because I was having to sand the part to fit !