I have both version of battery. I thought they typically have battery case that has contact behave like a spring that is flexible enough to accommodate both type. Please let me know if this is not the case.
Yes, some battery cases will not accept the slightly longer protected cells. It will depend on how flexible the springs are. I noticed on Aliexpress warnings "will not take protected cells" on certain items.
Additionally, and again depending on the holder, the larger width of protected cells(flat cable running down side of battery under wrapper).
I know the sub10$ solution in OP DOES take both protected and unprotected cells. Many of the flashlights I have, also do not take protected 18650's
This is exactly the method, I will order the PCB's with a laser cut stencil.
Checking the data sheet reveals an under voltage lockout @ 2.45V-2.6V so protected batteries are not needed. (LFP's can go down to 2V) It also has Over Voltage protection .
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Just to add to my earlier answer, there are dual fuses on the Output Side (F1 & F2) + the Batteries are isolated by the LTC4040. If you plugged it in the wrong way, it just wouldn't work.
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It is good to know that it has undervoltage lockout protection at 2.5V. It should be good last resort. I personally like to cut my battery somewhere around 2.9V ~ 3V. My main consideration is controller most likely employ trickle charging below certain voltage. This will make the entire recharge time much longer.
In the case of UPS, I do not think it is that critical to consider all these scenario. The likelihood power is out during trickle charging is probably small.
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Totally, I enjoyed researching the chip to find out the answer - batteries without the protection circuit are definitely more affordable.
I'll likely configure the LTC4040 to set max charge to 80%.
I'm in for 1.
Can we assume that for once it's not going to cost me an arm and a leg to ship to Sydney? 
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Hehe, shipping from Melbourne to Sydney is about $11 AUD for express.
So I think I have enough expressions of interest for this to be viable. I’ll order the parts and pcb’s tomorrow and that’ll allow me to provide a more precise cost.
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@dJOS Between you, @iharyadi and @lewis.heidrick , You guys need to start up a Hubitat-Mod Website...lol
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Hehe, I have a day job, this is just my hobby.
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I think I've done enough modding for a while.
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I think that's up to us. 
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Gosh Dang it!!!!! 
I went to order the parts and the blasted Semiconductor shortage has bitten us on the a$$! I can't get the required Mosfet from any of my 4 regular suppliers. 
There arent any similar spec Mosfets available either that I can safely substitute. Sooo Frustrating!
So this project is dead for the moment - I'll keep an eye out for the part returning to stock, but it might be a while.
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I have a feeling you won't see this part for years if ever.
I found what seems like a suitable alternative but I've only looked at the package and general specifications.
Vishay sira88bdp
It seems to be in stock in a few distributors in the USA.
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Cool, I’ll take a look and see if it meets the minimum specs.
I use digikey, mouser and RS - hopefully they’ll have it.
Aus to USA & Canada is $17.95 USD
If this turns out to be a "Go" It might be worthwhile to ship all those going to one country in a single package. Then one of the receivers could break it apart and ship the individual units domestically. However I don't know the cost of shipping the larger package.
OR, for those who don't mind a little soldering. Perhaps you could ship the boards without the battery holder. Again I'm thinking of ways to reduce international shipping.
I'm in the USA and am good at soldering (not volunteering at this stage). I don't know how difficult it would be to get all together, how many will really come through.
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The UPS's will only be about 50 grams each completed and I can ship up to 499 grams for $17.95 USD to the US and Canada - so if a bunch of you get together, I could easily ship one of you roughly 5-8 units for that price (including packaging weight).
After looking at the datasheets for a bunch of alternates, I believe you are correct - the specific model that should fit the bill is the SIRA88DP-T1-GE3.
EDIT: Scratch that, after some more research I found the SIR462DP-T1-GE3 is an even better match.
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A thought crossed my mind and I just wanted to ask to be sure.
Does this have any method of data output that would go through the USB cord. I am just thinking about something like if Hubitat could watch the battery and shutdown gracefully if it gets low or notifications if there are signs the battery is failing. I am still 100% in if not, but just wondering if that is anywhere in the relm of possibility. I am just not clear on how sophisticated of a bms this will be. I am also dont have high hope of it aince we are talking about lithium ion batteries.
