Here's an actual photo of the radio with the face plate removed, showing just the sides sitting against the chassis. The face plate drops into the notches while you hold the side pieces in place (put the GCAI dust cover on to keep that side firm), then flip the radio over and replace the screws. Once everything is buttoned up it's pretty solid. Maybe not quite as stiff as the 1 piece would be, understandably so, but with the battery in place there are no issues.
The vacuum purge procedure is not used to make the radio watertight, it's used to test the seals, notably on the R models. As long as the seals are all intact and you're careful with the disassembly/reassembly, there shouldn't be any issues. Note that only the R models are rated for submergiblity.
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