I therefore propose that we increase the proposed project card size from 2x4 to 4x4 which is a 'happy medium' which Phil suggests below. This also allows for Phil - VK6APH BPF board, A Jupiter GPS receiver etc. For 'Dead Bug' expeimenters like Bill - KD5TFD, he could use a 4x6 full length card and have 2" sticking out the front for experimenting without a card extender. It also allows us to get on with Motherboard design with a maximum vertical dimension of 96 mm. (case height between lands). I propose the motherboard height dimension be the the size of the 96 pin mating connector (3.7"). The holes in the connector will mate with standoffs for rigidity. I think a 4x6x4 enclosure would make a cute little radio!
I think that the ability to put the connectors, controls, etc. on the PCB is a huge benefit for repeatability and reliability of whatever this group builds.
You might want to consider designing the "motherboard" to accommodate the card spacing dictated by the slot spacing in this extrusion, then build a motherboard and at least one board of the proposed dimension, preferably with one or more PCB-mounted connectors or controls.
Next, measure the extrusion length required based on the stackup, including the mounting arrangement for the motherboard, to be sure you get the length of the extrusion exactly right. You don't want it to be too short, and you don't want a lot of "slop". Consider how the cards will be retained in the mechanical system design.
Then, order some extrusions cut to the known-accurate length, allowing for the tolerance of the cutting operation.
I won't mention how many times I've been bitten by not measuring, only calculating, an enclosure size.
Another thought: the motherboard could be sized such that its PC board forms the backplate of the enclosure. Downside is exposed connector tails -- or maybe that is a plus for having exposed test points?
73, Lyle KK7P