Ash blanket - or even just tin foil - would solve the ash/embers falling through the slots problem.
Second the observation on Leave No Trace - took a couple of Russian paddlers down Cat back in the late '90's (they paddled a bakraft, first time I ever saw one). They were indestructible and great entertainment, minimal camping gear but enough vodka so that didn't matter. Had to literally follow them around in camps and pick up their trash (orange peels, cigarette butts, etc.) but by end of trip they (reluctantly) would put 90% of their "thrash" in gallon zip-locks I asked them to use. Never could convince 'em that thousands of folks use the same camps every year and that should inform their ethics; did finally obtain some cooperation after 3rd or 4th night when they learned I'd been running Cat since '76... They remain among the top ten paddlers I've ever run with, could put their POS bakraft (similar to JPW Cutthroat or other straddle/paddle cat boats with minimal frames) on an eddy line and perfectly control angle&orientation to make any move needed.
Reading water is a gift. Some receive it early, most have to work at it, and a few will always think muscles can take you wherever you need to go.