Coincidence or fate!?!? LOL just kidding 😛
As you say the antenna covers aren't really a problem but the rear panel is another matter; there just aren't any about, and had you not told me about the "small parts" I wouldn't have known. I had a look at the link you posted and for a rear panel with "small parts" they want $65 USD which I don't think is too bad, but they're temporarily out of stock...bloody typical!
To be honest I don't really want to replace these parts myself at all, I had a bad experience trying to take apart an iPod 4G once and don't fancy a repeat performance, I've looked at a couple of places that repair iPhones but aside from the high prices for parts and labour they don't have the parts in stock either.
It may be worth buying a broken donor iPhone from eBay, but then there is no guarantee that the condition will be any better than what you already have; everything listed as mint seems to state that the items have a few minor marks and scratches in the descriptions, which surely that defeats the object!?