The Smart 8 looks like a pretty decent light. I haven’t seen independent testing of it, but it claims a lot of output. Probably enough for a 5x5, yep, but dimmable to use in a 4x4.
What are you using to measure DLI and PAR? Because if you’re running that Smart 8 at the lowest 40% dimmer setting (~300w), it should be as close to your canopy as possible. 300w is good for veg, but it ain’t much light in a 4x4 for flowering. If you have a way to measure power draw, I would confirm that it’s actually dimming properly, cuz 100% is claimed to be ~760w. Even at 80% (~600w), it shouldn’t be too much light for flowering a 4x4 at 18" above the canopy unless your plants are unhappy. No reason to have to use those diffusion silks!
The DIY build depends on your budget. I’m using these DIY lights in a 4x4, dimmed to between 550-600w in flowering depending on plant health:
and I would still recommend them for a 5x5 if you run them at the full 720w. Although you can build with 1x1x1/8" angle aluminum for cheaper than my slab construction.
The Bridgelux 2700k strips used in that build have a lot of far red and very little blue, so they’re damn near perfect for both veg and flower since we now know that far red makes plants grow faster. Also, higher PAR can be used to keep plants short even when there’s very little blue in the spectrum.
If those strips are not your thing, there are some newer high output Bridgelux strips that reduce the build costs cuz you use fewer strips, but require hanging a little higher. Happy to discuss if you want to PM me about it when the time comes. 