Because a suitable efoil board for a beginner needs to have volume (safer to start with in case of failures, electric box waterproofness is less a pb, softer water touch downs) AND be compact.
How much: your weight kg plus 0 to 20L = board vol in L you 95L to 115L. Don’t forget you also carry 15kg of batteries, a 5 kg foil, a 2 kg motor.
Compact means:
- short : easier to manoeuvre (less swing weight in turns) and transport
- thick to welcome your batteries
- wide and thick for early take off.
Since you need a square and thick tail, the Mistral seems suited for that, slightly cut or not, I suggest cutting at the nose somewhr below and above the Mistral M to maintain the tail integrity. @Manu post above explains how to fix the US rails.
Everything is explained here where you have interesting board shaping projects (blank cost 100-130usd pre-cut)
Well suited (short, wide, thick) and cheap for our needs are the second hand (Formula) kite race boards. In the 2-page (+ 1 page for outline) list provided by PM, look at 8th column (Volume) to match your weight. The shorter (less than 1.80 or 6’), the better.