if long exposure is your objective get a variable ND filter.. save you all the pain in fabrication.. Also those industrial glass are thick and may be scratch so it could cause degrading of image quality.

Of course, there's the Lee filter Big stopper, 10 stops.

Lee Big Stopper 10 Stop. rm7xx

i tried with not one, but a few Welding Glass (my line of work let me get my hand on them)....
1. some has low cut off of 650nm, u get funny color out of it. aka Color Tint. like Red, or purplish blue. in other words, its like IR filter.
2. it terrible distort the image quality due to its thickness(i suspect) when used with ultra wide angle...
3. corner is crazy dark/heavy vignetting. i think due to the thickness issue too.
4. i think welding glass is around 4stops to 6 stops, never measure it. but its doesnt slow down to 10 stop like Lee.

my advise. skip the idea, grab a variable ND for up to 9 stop, or B+W has a 9 stop (ND1000, around 500-600) filter too. or go for LEE for minimal color cast/tint.

Seems like you're on a budget (since you're considering the welding glass), why not grab the Hoya ND400 (9 stops ND)? It's cheaper than B+W and Lee.

take a look at LightCraft Workshop Variable-ND as well.. gives a good 8-9 stops, though it gives a 10 stop, but at 10th stop, there's a distinct dark cross going across the image.

Hey u interested? i have a hitech pro stop 10 for sale

https://forum.lowyat.net/index.php?showtopic=2719449&hl=