FingerlessHackWoodworker – welcome to this thread! Love the monniker, hope it's not completely true
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The answer to both your questions is, no neither of these features are critical. Roadrune has it correct, the critical cabinet details are the correct internal dimensions, the external cabinet width of 9", and the speaker & port locations. The rest is up to you.
I do worry about the thickness of your front baffle. Your drawing shows the upper front baffle as 3/8" thick and the lower part as ¼" thick. I think it should be at least ¾" thick overall for two reasons. (How thick are the other sides of the cabinet?)
- The ER18 woofers are heavy (4.73 lbs each) and their mounting frame flange is 0.2" (5.2 mm) thick. When you cut the recess to flush mount them, the remaining wood may not be enough to hold them securely.
- A front baffle that thin will probably will vibrate audibly, even if the screws do hold the woofers.
When I made the drawings, I was imitating the look of these cabinets:
The photo shows Salk Veracity HT2-TL speakers
https://www.salksound.com/ht2-TL - images.htm. They are the same dimensions and shape as the ER18 MTM cabinet. In fact, when Dennis Murphy designed the ER18s, he used an unfinished "test mule" cabinet for the HT2-TLs that he had in his basement.