Got around to working on this again yesterday. I can see the pieces aren't perfectly sized/aligned, and I think it might be a bit narrower than I would like, but I did the best I could with limited DIY skills and hope it's all close enough when placed onto the bracing. This perimeter bracing has 4 drilled holes at the back and sides for hosing etc. so I can't use the full 4 inches of bracing except at the front. On the right side I also had to make space for the OHF that goes across the tank width there.
Cutting and putting on the plastic webbing with zip ties was a time consuming process but it's strong as heck, no Arowana gonna break through that screen (but it's more forgiving on the head than say glass or polycarbonate). Plus given the small pebbles loaded into most of the PVC, the whole unit is considerably heavier than the aluminum ones currently on the tanks. However the outer frame will still need to be lashed down to the tank stand for max safety.
Next I need to fix handles onto the lids and then figure out some kind of latching system at the front of each; thinking about velcro or maybe simple rope handles and upside down hooks.
After the outer frame goes onto the tank I'll put the lids on and use several looser zip ties to be securing 'hinges' at the back of each lid.
Not beautiful but should be functional and secure -- if it fits properly. If not I might be able to adapt it for the BBXB's tank, which has 5" bracing.