The original BBC units were built from 12mm Birch ply with Beech fillets. The size of the enclosure is set primarily to fit the 5" bass driver with the tweeter occupying a smaller space in the top of the baffle.
A quick survey of available driver units led to minor upgrade replacing the 5" originals with currently available 6.5" bass drivers and 1" dome tweeters. The box would need to be increased in size to suit. To keep to the proportions of the original design, so it looks right, all dimensions were increased appropriately. I would need some boards, preferably Australia hardwood about 8" wide for the box.
Down at the local boutique timber yard a couple of beautiful pieces of Spotted Gum were found. 8" wide boards 1.5m long, oh yes, and 25mm thick. Spotted Gum is historically used for heavy construction, and still commonly for axe handles. It is tough stuff. I figured that as a speaker box material, in 25mm thickness, it should be virtually resonance free and not need any bracing. To keep the external appearance in keeping with the original LS3/5As the front and rear edges of these monster boards would have to be rebated to 12mm where the back panel and front grill frame fits in.
Preparing the boards with my available tools was a little challenging. I chose to join the corners with plain 45 degree miter joints. As the boards were 25mm, the width across the miter is 36mm which was too long for my 45 degree router bit. My only (power) cutting tool is my 7" hand saw, so I was going to have to use that. Setting up some guide rails to ensure straight, square and accurate cuts was essential, as was a new 40 tooth blade for the saw. My first couple of cuts were nice, but not 45 degrees. The saw would not adjust out that far, missing by a couple of degrees. Not good. The solution was to disassemble the adjustment screw and replace it with a smaller screw that allowed the angle out to 45 (determined by trial and error). Once correct adjustment was found the dodgy screw arrangement was locked in with a cable tie around the slide. I hope you have a better saw or machining equipment.
With the sides tops & bottoms cut it was time to route the front and back rebates. With a nice chunky router and new 19mm square end bit a wide rebate for the front and a shallower one at the back can be cut. I was amazed at the quantity of sawdust this process generates, the shop vac got a big workout.
Next the front baffles and rear cover can be cut. The front baffles have to be routed to take the bass drivers from the rear and the tweeters from the front. There may be a way to jig this and get perfect circles, but frankly I couldnt figure it out. I just marked the circles and slowly guided the router around. The result was quite acceptable.
Next the box can be assembled. After some failed juggling I realised I needed some sort of bracing to hold the sides in place. I settled on inserting a piece of 12mm square hardwood in each corner and drilling to take screws internally in each direction. Clamping with the baffles and rear panel in place was then used to keep things square while the glue set (high strenghth epoxy).
