I wasn't going to critisise it at all I was actually just going to suggest instead of decking would it be possible to turf it for them, it would look really nice aswell, if you can't do that it might be worth building a small frame and laying a small couple of squares of turf and a little mud patch so they can dig it, inside a ply frame if you see what I mean so it would still be contained but a little patch to keep the nails down/forage, you can do that with shallow sided box filled with hay aswell and hide some treats in the hay.
I actually just today planted a couple of seed trays with turf for them.
My wife suggested turfing it all instead of decking, but I decided that it would end up in a right mess, as it is not practical to strim it if the bunnies don't keep it all trimmed for us.