I had to read this out a few times before getting my head round it. I am slow. Apologies lol.
I have had a couple of thoughts as well for your install:
1. Any particular reason that you want the amp to be removable? You mentioned that you wanted it for the bootspace when you flatten the seats. But, I reckon that your 2 amps might be able to be mounted upside down, like in Mr_Spock's build neatly underside where you were thinking for the mono amp. that way, all cables are neatly routed to it and you can still access it for tweaking the parameters if needed. I am asking this as you mentioned the sub enclosure will be permanently in the car. The JL Audio website should have the dimensions of both amplifiers if you want to measure that space in the car to see if it will fit.
2.One other option that we have not considered is that, with the 4 channel amplifier, it doesn't need to be close to the subwoofer. You could have it mounted inside the cabin if you wanted. e.g Under the front seats or even where you used to have your spare tyre. You could have a very tidy install of the 2 amps in the tyre well with all the cables out of sight. It would have the advantage of keeping all your boot space nearly as the enclosure take a very small amount of it
The reason I am thinking of alternatives is that, the amplifier will get hot with prolonged use and you need good airflow to it to keep it functioning well. With your idea of the mounting position, if I gather correctly, will be enclosed or not? Just my thoughts about it as I reckon it might be a pain to get it in and out when you need the bootspace with the kids etc.
Your plan will definitely not cost much and will be a fun project to do at the same time. You have thought it through with the weight etc as well. Thankfully the sub enclosure isn't that heavy. It was lighter than my old 10" sub with its box! I think it's well worth the money.
With regards to the custom cut pieces, I reckon they would look very nice when illuminated. Would definitely be the icing on top of the cake then. If you are going down that route, also consider making it in fiberglass and painting it as that would be even better for the amp rack