How to add a fuse/breaker to an extension cable
Fatal flaw: You can't use extension cords coiled up!
This entire product's very design is faulty.   Obviously the design intent is that you furl out just as much cord as you need, and then leave the rest neatly on the spool.  But you run smack into NEC 310.15(B)(3)(a):

A spool counts as a raceway.  That spool could potentially hold 11-20 wraps of wire.  Hot and neutral count but ground does not, so that's 22-40 wires.  The table calls for a 40% derate - very extreme.  Normally, wire ampacity is limited by several other derates, fortunately this does not stack with them, it derates off the &quot;theoretical perfectly cooled&quot; ampacity.  At 90 degrees C, that is 18 amps for #16. 40% of 18A is 7.2 amps.
(and we're being generous here; if we hold #16 to 60C temp, your limit is more like 4A).
And when you violate that, you get this.

As you can see, this product is a total loser from the day it was drawn on the back of a napkin in a bar.
And that's where the CE rating came from.  These epsilon-minuses had lined up the factory and faked their CE rating before they even realized you need a UL listing (or other third-party Nationally Recognized Testing Laboratory) to sell in the United States.
UL told them to take a hike.
CSA told them to take a hike.
ETL (by golly, when there's a marginal product in need of approval it always seems to be ETL), approved the product on condition of adding a fuse.  So they had two choices: be yet another Chinese seller shoveling junk onto wish.com, DealExtreme, Amazon and eBay... OR, be in Home Depot.  So they bit their lip and said &quot;alright, fuse it is&quot;.
Hey, the NRTLs certify safety, not product quality or fitness of purpose.
Then you decide you want to modify it.
Statutorially, this is modifying equipment contrary to instructions, and is a violation of NEC 110.2 and 110.3(B).
You could do it (illegally) by terminating the cable with a junction box* this is illegal, and fit a furnace-style fuse (a fuse and switch or socket in the form-factor of a common receptacle).  You see this all the time around construction sites, usually made of a 4x4 steel junction box and a domed cover. I don't like it because those knockouts are actually pretty fragile, as will be the fuse and cover in your case.
Bottom line is that a &quot;reel up extension cord&quot; of this type is a very concept which is technically infeasible.
The only way to make that work would be to use 90C-rated #10 wire.  At that point you're coming off a 40A derate at 40% giving 16A.  Or, make the drum large enough that the wire only wraps around it 4 times - that would require a 2'4&quot; drum in your case.
#10 wire is a fair bit more expensive and would more than double the cost of the reel.  But if I wanted that, that's exactly what I'd do. Get somebody else's cord reel that lets you use your own cord... and use #10 cordage.


* By the way, now watching this video, the builder made a very poor choice of cable clamp. They make proper strain-relief cable clamps made for cordage (that one is made for Romex). And you have to ground to the metal box, which then takes care of the outlet grounding. And the person recommends a &quot;screw-and-clamp&quot; spec-grade recep but doesn't explain that adequately. Amateurs!!!