Rabbits A new cage for my lionheads

So I have two lionhead rabbits Monty(as in Monty Python) and Jessica(as in Jessica rabbit(duh)). Currently they live in a regular cage I got at the petstore(wire with a plastic bin bottom) and an hexagonal exercise pen with a mat designed for it as the bottom which they have access to at all times so they can go between the two. They seem pretty happy with this the only problem is they tend to eat the mat exposing my (new) carpet which since they don't do a great job getting to the litter pans(one in the pen one in the cage) all the time I don't want them ruining. That's when I discovered guineapigcages.com and the idea of C&C cages. I have now designed what I would like to do and it looks like this in concept

WWWW WWWW WWWW
W S S W
W S S W
W S S W
WSSSSSSSSSSSS W
W S S W
W S S W
W S S W
WWWWWWWWW W
W W
W W
W W
W W
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W W
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basicly a 3W X 4L X 2H with a 2X2 ledge\small room in one corner so they can hide if they like(Jessica does she's a scardy cat). There will also be a ceiling in the ledge area so they can't hop out. Now my questions.

1) the ledge should i change it to just go along the back wall?

2) connecting the panels do you advise using the connectors that come with the cubes or using the zip ties? I know I need my ledge to be sturdy since rabbits are a little heavier than guinea pigs. Mine are about 5 pounds Jessica is actually bigger. They also tend to run around a lot(they are rabbits) especially when Monty chases Jessica to do what rabbits do best(not hopping ) and will often hit the walls.