Thanks for all the kind replies.
ive glued up the cabinets in sections, with the central parts glued in stacks, and the wider elements separate for now. this is to allow sanding of the recessed areas.. ive almost finished that phase of the sanding. after that i will glue them up completely, apart from the top panel, to allow continued access, and sand the whole speaker, probably with the top clamped on temporarily.
regarding the circle jig, my idea was to rout the outer circle down to the depth of the lip (circa 1cm) then rout the inner circle through, apart from a few tabs around the circumference which i would leave a couple of mm of wood, to hold the centre in place. after routing i would cut out the tabs using a hand saw or stanley knife.
i wasnt sure wether to rout the clean outer edge first (mm depth by mm depth) using the downcut bit, then do the rest with a decent straight cut bit, or, rout out the outer edge a few mm inside the final edge using the straight cut, then use the downcut to slowly enlarge the hole out to the final dimension.
i have been advised the second option, but to my (inexperienced) mind, the first cut with the straight edge risks weakening/damaging the end-grain of the ply for a good few mm either side of the cut, which could compromise the cleanliness of the finishing cuts with the downcut spiral...
what do we think here?
the carpenter (yes im using the term loosely, im translating from the italian "falegname".. these guys do everything from window frames to furniture restoration... there is also an "ebanista" in town, which i believe translates to "cabinet maker" but ive no idea of his skills) suggested cutting inner circle with a hole saw, and then use one of those bits with the bearing on ( im not sure of the name) to run around the edge of that to get the lip.. however a) the roughness of the inner circle would cause imperfections in the edge i am following it to produce,and b) ive only seen those types of bit with a straight edge, which i know is inferior for producing a clean edge..
finally, if i buy a decent straight bit and a spiral downcut bit, and use them with my circle jig, they will be useful for basically any other project.. useful given the cost of decent bits.. if i buy a specific bit for "following" a hole, it will only be useful for other projects where i need that exact size lip again..
thanks for the pointers re. bits.. for the spiral bit i was stuck deciding between a whiteside standard downcut spiral, or a freud o-flute downcut, which have basically the same price. One guy on the diyaudio forums swore by the o-flute as he said it makes a cleaner cut on plywood.. ive read conflicting stuff after which says the more flutes, the cleaner the cut, so im confused...!
my main fear is the unusual nature of the wood to cut.. its all basically end grain, with a change of direction every 2mm, and any number of potentially weak laminations or areas of thicker glue... it will be finished with a transparent coating, so anything more than small single lamination damage will look very ugly.