I have used a wide variety of nestboxes. There are pros and cons to every style out there.
Yesterday I was out washing up my boxes to let them sanitize in the sun and though I should talk about nestboxes with you.
The Style that I prefer
The high sides are what my does (90%) of them will jump over to get into the box and the height seems to encourage latched on kits to unlatch into the safety of the box. Kit safety is the biggest reason for nestbox design.
The Step away from lidded Nestboxes
Straight Sides Lower Front
All Sides Equal
A white plastic box, I can't remember what the plastic is called. I told the person who designed it that I would give it a trial run for a discount. HATED that box. Wet kits, no drainage, I use it for keeping spare fur from the does. So it's still worth something to me but not as a nestbox.
Mandarin orange crates. These work really well for polish. It surprised me how well they worked. Kits stayed in, perfect size to keep warm. If I still had polish I continue using them, but the mice my son raises use them as hidey holes now.
Other world-wide designs
Other people use old cat litter tubs or plastic square pails.
I've seen heavy cardboard boxes used successfully.
There is a world out there with creative ideas for nestboxes. Watch your rabbits, analyze the results of your builds and let them do their job. Be as helpful to them as you can be, that is your job as a "rabbit herder". :) Then stand back and let them be the mommas you know they can be. :)