These things are legal, as they're not threaded, shouldered or otherwise designed to accomodate a tube. To make them into suppressors, you'd have to either turn down the body leaving a step and then thread the front for a cap in some fashion, or externally thread both ends for collar nuts to retain a tube. One could weld a tube fore and aft, but that's true of any brake.
That said, aside from a couple of them that would be OK for .22 LR (like the one bottom-center in JMorris' post that essentially is a rip off of the Gemtec G-Core), in most applications monocores just don't work that well, and especially not very simplistic designs like most of these "brakes". I spent a lot of time, money and effort playing with monocore designs, finally had to accept that there's just no way to make them perform like stacked baffles.
I built a pretty decent 30 caliber can from a D cell tube. By the time everything was paid for I might have $120 in materials invested. There are precious few decent 30 caliber cans under $500, so the cost savings are significant.
I don't know what sort of parts you purchased, but a threaded Ti D cell tube will run $70-$100 by itself, and then you're $20-$35 ea for machined baffles (7-10 for a decent .30 rifle can), plus end caps. Yeah, you can put together a stainless or aluminum housed freeze plug can for a bit over $100, but expect to be more in the $300-$400 range with Ti tube and good stainless baffles.
My 1.5x8" Valkyrie and 1.5x9" Furtivus have almost $100 in raw materials each with gr 9 Ti tube and 17-4 bar stock for baffles, end cap, rear mount and muzzle brake.
If someone were to buy a few remade cones and then bore and clip them, they could make something essentially functionally the same as the best of the high end cans. The cost savings of a Form 1 can are nothing to sneeze at.
I respectfully but emphatically disagree. If it were that easy and that cheap to do F1 stuff that's "just as good as the high end cans", there'd be little market for the production stuff. Even with the "best" commercial form 1 bits, there's a lot to be desired compared to professionally manufactured cans. It's not just about dB reduction; weight, size, finish, durability, POI shift, mounting system. If one is happy with their screw-together Ti tube & 316L stainless direct thread can, fantastic, but it really doesn't compare to a production model with fully welded 17-4 SS or 718 Inconel baffle stacks, QD or taper mounts, etc. that has been developed over multiple iterations with minor & major changes to aperture size, clip, spacing, blast chamber size, brake style and so on.
Don't get me wrong, I absolutely encourage the F1 crowd. And properly researched with a decent budget, some of them come out pretty good. But ultimately, unless the builder has decent machining equipment and skills, the F1 will come out behind in more categories than not compared to a commercial model.