Well, I do not have any practical experience with building a platform either, but some ideas come to the mind, based on experience from line diving from a boat:
It is rather easy to bump your head on the platform when you are ascending, so it is better having the descend line a meter or more apart from the platform. If you plan doing a platform with a hole in the middle, then the opening should be rather large to allow for a simultaneous ascent of at least 3 freedivers (1 freediver + 2 safeties), better even more. That may require rather huge construction, so having the descend line(s) on the side(s) may be a better idea.
Having nooses, seats, saddles, benches, or steps underwater at the side of the platform may allow for better relaxation between dives than having to climb on platform, or just holding on it.
Enough of handles or lines to hold on immediately before the dive, both for the diver, and the safeties (or coaches, judges, spectators) is also always welcome.
A compartment and/or fixations for putting some spare equipment, drinks, food, documents, devices, lines, weights, first aid kit, phone or radio ... At least one compartment should be waterproof. Well, you may not need all that if there is your boat close too, but having the possibility to have some things close to you may be an advantage anyway.
If you plan using a counterweight system, you will need to build the platform symmetrically, relatively large and quite stable
EDIT: Make sure there are nowhere protruding any screws, sharp objects or edges. Every of them will pose a serious risk of injury and/or wetsuit damage. Especially if there are waves, it is very easy to hit yourself. That's why using soft coating for any hard parts may be a good idea too.
