I'm a bit confused by the FE127e Fonken port and spacer dimensions - the plans show 3" ports and spacers of 1 1/16", the notes suggest spacers of 1 1/32" but with an internal box height of 13" and 3 ports, shouldn't the 4 spacers be exactly 1" ?
If you're working from the earliest version of the drawing, note that all panels except the baffle are indicated at 12mm (1/2") Not long after this drawing, we normalized to use of 15mm material for most enclosures, which we found sounded much better. We've also used 3/4" bamboo plywood for at least 1/2 a dozen pairs.
I'd have thought that Dave would have revised the sketch linked on his site by now, but since I've always redrawn my own to create cutting plans and CNC patterns as needed, I hadn't noticed .
Changing to thicker materials and our choice of joinery methods would obviously affect at least one dimension of all cut parts - the important thing would be to maintain the internal volume, and the height / length of port slots.
I've built dozens of pairs of this design and its numerous descendents - as a personal aesthetic, I prefer the spacers between the slots to have the same overall height as the combination of those at the end and the top/bottom panels. As noted above, these would be affected by the thickness of material used.
Using 1/2" as drawn, I'd make the middle spacers 1" each and the end 2 1/2" each. Yes, (13-(3*3))/4= 1" but that does not include the top / bottom.