Agree with Doug and Jakub. And to add a few thoughts:
1. I had the same feelings as you with not wanting to rest directly on the heating element. I found a stainless stand-off. (mine is Stainless Steel False Bottom for Brew In a Bag (BIAB) - 14.75'' Diameter )
2. I built mine with a PID from...
The link you provided says the expected length is 14.25"
I measured my setup with 5500 element and (different) mount to be just under 13.5" (see photos). It would be close in your pot. Is the 13.5" pot diameter ID or OD?
Element:
https://www.brewhardware.com/product_p/element5500_ripple.htm...
Careful with having it all with a rigid basket. To do a 5 gal batch in a 20 gal kettle, the ideal basket would have very low clearance/dead space below it. This very well might limit space for an element. If you later wanted to convert to electric, it might not be compatible with the same...