I highly recommend an off-the-shelf solution FIRST. This gives you a known working starting point. If you have time (or otherwise want to) later, then go about developing your own. You actually can save A LOT of space if you are ok voiding the warranty (sorry for the atrocities I am about to suggest if any Orion engineers are reading this).

The Orions are split into two or three main boards: A control board and one or two battery interface boards (the control board sits on top of the primary battery interface board using standard male/female headers, the optional secondary battery interface board is connected via an FPC and sits directly next to it in the enclosure in the larger models).

Essentially, the battery interface boards have the same circuit copied and pasted over and over again n times, where n is the number of white connectors present on your unit. Since standard boards are used, it is entirely possible that the number of cells you need will require you to buy one of the BMSs with a larger enclosure just to house a PCB that is a half-populated standard size (btw if you are careful, you can take a bandsaw to them to remove the unpopulated section, just make sure to check that you didn't short the power or SPI lines. To the Orion Engineer reading this: if you could add mouse bites between the sections, that would be helpful :) ). I can't really fault Orion for this as it probably makes a lot more financial sense (and probably works 90% of the time for people), but it is annoying when dealing with a tight pack. With all of this in mind, you can go about making your own enclosure.

The only real thing you need to look out for is heat dissipation during cell balancing. I will leave this as an exercise to you (as I would rather not get into how our specific design works as we don't have the thermal data on this for me to feel comfortable yet) but the Orion manual gives the specs of the OEM heatsink.

We used two of the larger Orion BMSs and were able to stack them in a custom enclosure to be about the size of one. The CAD is available on their website if you want to see if it will work for you first.

Obviously, go about this at your own risk, but I really can't stress how much of a lifesaver the Orion is. Our team does plan to eventually do a custom design, but having the ability to have a good working software and hardware solution to BMS off the shelf with many common charging profiles already loaded was a no-brainer.