Foam core ply is practically impossible to get hold of in the UK, so I've been toying with the idea of some alternatives for a lightweight solution. This is driven by the usual pushing-middle-age issues associated with back/shoulder/neck problems.
I've come up with the idea of using 3mm hardwood ply sheets, 6mm ply strips to form internal bracing between the 3mm sheets (such that it forms the outer edges of the cut boards), with the remaining gaps filled with 6mm Depron foam sheet, which is a fairly incompressible dense yet lightweight foam used for thermal insulation and soundproofing.
The idea is to make up a composite 8'x4' board using this braced laminate construction according to the cut-sheet design, such that when I cut the board to size I'll be slicing along the line of the bracing.
So I'm after a sanity check - obviously this is a lot of preparatory work and it may be a complete failure, but I reckon it could save something like 40% of the weight on a standard 12mm ply cab. The woofer baffle would have to remain solid 12mm for stability's sake, and I think I can get away with 9mm internal bracing. I'd be looking at an Omni10 or 10.5 for simplicity's sake and to road-test the idea.
Any comments on practicality or has the sawdust finally rotted my ailing brain? Give it to me straight guys.
S.
Foam Core Ply Alternatives
This is just my observation of foamboard, I've printing on this stuff for 20 years. One problem I can see is in a damp enviroment it will warp also it would have to be thicker to maintain strength. Maybe look at Gator board wich has a plastic exterior rather than the paper and it's about the same weight. But I wouldn't want to be moving these around alot, unless reinforced with say Fiberglass resin. Cool thought though 
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4 x T48 24.5" 3015LF
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4 x T48 24.5" 3015LF
8 x 112 Otops
NEXT 2 x DR280's
I built a set of 10.5's for a small PA with foam core ply.I had sticker shock when I paid about $120 per 4x8 sheet.
I first cut all the peices and then removed an eighth of an inch from all edges and filled with PL just before fastening in place. They have been in service for over a year and about twenty gigs and every time one of the guys picks one up and place it on the stand they thank me for my effort!
So it was worth the effort but I'm not sure if I could say that if I had to build the ply first. That being said, how about end grain balsa for a core. It has a higher compression streghth than foam and it may be available in your area.
I first cut all the peices and then removed an eighth of an inch from all edges and filled with PL just before fastening in place. They have been in service for over a year and about twenty gigs and every time one of the guys picks one up and place it on the stand they thank me for my effort!
So it was worth the effort but I'm not sure if I could say that if I had to build the ply first. That being said, how about end grain balsa for a core. It has a higher compression streghth than foam and it may be available in your area.