If you insist on using the word "shield", it is clearly simpler to go vertical, as intended by the makers, than horizontal, and I don't think the need, real or imagined, of access is anywhere near as bad is you think it might be. The worst that can happen is that you may be obliged to put dummy long-pin headers under short-pin boards.
The idea of spreading them horizontally is just absurd, and may even cause self-inflicted grief leading you to actually need the multimeters and logic probes that may otherwise have been unnecessary.
If this is actually just to demonstrate an intellectual exercise, the components on shields are all available individually or on breakout boards. These could all be soldered onto a perf board, along with a Pro Mega, which has standard pin-spacing. Otherwise, read reply #2 again - last line first.