Make connector less fragile, move inside top/bottom enclosure

[scottbez1]

The connector slot on the enclosure side pieces has historically left a thin sliver of material on the corners of the side pieces that was prone to breaking during shipping, especially for acrylic.

This change should make the entire connector system less fragile by:

• moving the connectors from outside the enclosure top/bottom pieces to inside, which removes the thin sliver previously left by the slot cutout - circled in red below
  • (there is some risk from this change that flaps will catch on it, so I'll need to validate this change in practice. I added a fillet to the outer corners to make it less likely that this will scratch flaps if they do end up in contact)
• adds fillets to the connector slots on the enclosure pieces and the connector itself, circled in green below
• increases the overlap between enclosure/connector to reduce thin areas