Is that rigid HDPE?
Yeah. It's the same stuff they make plastic cutting boards out of. If you have a cheap white plastic cutting board in your kitchen it's probably HDPE. They use it to make 1 gallon milk jugs in the USA (not sure elsewhere).
It's very tough stuff.
I've tried ABS and a couple of different acrylics.
ABS is used a lot in consumer products because it's just about the cheapest possible plastic you can use and still get something reasonable. It's easy and gentle on molds. But you won't see it used in serious tools, for example. For that they use fiber reinforced nylon, which is brutally stronger. Good quality tools like Makita and Milwaukee use reinforced nylon. They use that nylon fiber stuff in firearms like Glocks to give you a idea. The plastic string they use for weed eaters is plain nylon. Pound for pound the composite versions exceeds steel.
ABS isn't something I would build anything with.
Acrylic is "OK". The best things about it is that it's clear and cheap. I wouldn't build anything using Acrylic unless it's only job was to look pretty.
If you want something clear then polycarbonate would be awesome, but I don't know much about welding it. Polycarbonate is what they make things like bullet proof glass out of. They also use it for soda bottles, which believe it or not are extremely tough. Those paper thin walls need to hold back around 50-100 PSI.
Will the HDPE flex a little without cracking?
I would have a hard time making it crack with a sledge hammer, I think. Unless it was extremely cold. It's very impact resistant and can bend and be shaped by heat easily.
HDPE is "High density Polyethylene". They will sometimes use this type of plastic for "Living Hinges", which are thin sections of plastic that act like a hinge.
Think of the caps for plastic ketchup bottles or caps for shampoo bottles and things of that sort. How they have a lid that pops off and pops back on. The lid and the cap are usually one piece that is connected by a thin piece of plastic.
It takes a lot of twisting and folding and hinging to get those tiny bits of plastic to break. The use polyethylene and polypropylene for those types of applications.
Here is a guy building a 125 gallon RV water tank with HDPE:
That's worlds different from acrylic.
I have a Tap Plastics a short ebike ride away, and they have a pretty big selection of scrap pieces for next to nothing (actually nothing during the pandemic, since they were so busy, they'd just said to take it).
Sounds like a good plan. You'll want to check out HDPE, polycarbonate, polypropylene, and polyethylene especially. Probably can ask them for advice on what to use and they will know a hell of a lot more about it then I do. I just got what I know off the internet.
found this: https://www.mcmaster.com/plastics/