Good to see there’s going to be a 5th custom evolve gen 2 in London…
As far as mounting the focboxes inside the deck, unless you have a custom enclosure, you will have to cut a little bit away for them to sit nicely on the heatsink, (just cutting away a little at the carbon that adds a lip by the heatsink)
a 10s5P battery (im asssuming is 18650s) will have plenty of space in there i myself am using a 10s6p of samsung 50E 21700 cells (but the rest in London doing this use a 10s7p battery), but then again all i wanted was range rather than power, i would recommend making a battery riser out of PVC plastic sheets (I literally took some out of a scrap bin from somewhere that works with them near me).
If the batteries are all fully made and working (with a bms on them) then you could easily just use some adapters (deans or xt90 to xt60)(xt60 splitter) to power both of the focboxes without any soldering, if your focboxes have the correct connectors for your motors your also in the clear, the only thing that could do with soldering would be a charge port(might be helpful) but since you will only really be using low current through it your local RC/Electronic store might be willing to do it for you for a price (e.g.£5)
All this really depends on how complete your collection is, and how much wiring is required.
I myself am looking at getting the tools required to make a new board on my own (soldering station, spot welder), but that wont be till the end of this month or the following month.
All i will say is while the board isnt as nippy as something shorter like a boosted mini, it makes up for it in a nice stable deck that is good even to 36mph.