When I built our county portable multi radio cabinet for field interfacing multiple channels, I did basically what VUL said except I did the mixing with a small PA mixer with a built in amplifier. It was a 1U 19" rack mount cabinet. I just built the radio speaker output cable for each radio with the voltage divider as part of the patch cable. Worked like a charm with very little effort or expense. Good audio quality and easy control. Plus being made by Shure, it was built like a tank. And if it ever died for some reason, it was a standard product which can be swapped out in minutes.
I did like having the control aspect as it was easy to mute any or all signals in my local speaker but yet not change anything going to the Harris interface controller.
You guys are not following me - whatever we buy has to be small like the Fleet Radio Products 4 port combiner so it can fit inside
either a havis or Troy console and not a bunch of parts that's cobbled together - local gov purchases and higher archy have some
requirements besides the common sense aspects of installing this inside a console.
Sorry...I didn't realize using a commercial product made for what it is doing and installed properly was a cobble job. I really don't need a lecture for offering my experience and knowledge for free.
Now as to your question, why not give the size limitations, actual functionality based on theory of operation of what you need instead of screaming about something specific that is no longer made. I am not gonna do your homework and start looking up a bunch of stuff on Google when you can do that yourself. Especially for free information that, if you know what your doing, you can come up with yourself. Or call a radio dealership that handles commercial communications applications.
As to the purchasing, my county told me what they wanted, I gave them an estimate, and they accepted my bid. I do believe that is the normal channel for bidding and winning a contract. No cobbling there either.
I will follow you if I can truly be of help, but I won't do all the work to hand out free advice.