And...drum roll, please...my UK 45 degree angled hex Allen wrenches FINALLY arrived.
https://www.ebay.com/itm/183036366341
It was a second order because the first order, ordered on May 15th, sat in a Chicago warehouse for 40 days. I could see it got there from the UK. I queried the seller on Ebay after two weeks of inaction - they said give it time. Well it never moved - perhaps somebody did a "five finger discount" and stole it - or it is buried like the Arc of the Covenant in the Indiana Jones movie. Not sure if the offending warehouse was US Customs, or US post office, but after six weeks of inaction I got a refund, then reordered the exact same product from the same Ebay vendor - and the second time it took only 10 days to show up.
It is the perfect solution. It fits, it doesn't hit anything, its range of motion when inserted is easily enough to break loose the fill plug (but be careful not to bang it into and scratch the anodized aluminum of the DIRS) and by extracting and reinserting the wrench unscrew the fill plug. Therefore, I hereby declare my Per4mance DIRS installed Challenger differential fluid fill plug loosening and tightening problem resolved - for $25 - plus I got 9 more angled hex wrenches to go with the 8mm for the fill plug.
View attachment 79396
I'll mention one caution about draining differential fluid with the DIRS installed. That drain plug access channel straight below the fill plug with the Allen wrench inserted in the above picture - to get to the drain plug that channel is narrow. I realized after I unscrewed the drain plug with a long straight 8 mm Allen wrench that IF while trying to retract the plug the drain plug fell off the end of the Allen wrench it might fall sideways, or get angled, in that channel and then be a total PIA to get back onto the end of the Allen wrench to be able to rescrew it back in. I think almost impossible to get it back on the end of the wrench fishing around in there. If so it would require disassembly of the DIRS. So I didn't try to retract the plug - I just backed the drain plug slightly out and fluid started draining just fine, then laid the wrench with plug securely on its end down inside the channel, and let the fluid drain to my heart's content. Rescrewing it in was easy. Words to the wise...