I recently adopted a 2 year old Dutch Dwarf, Oreo, from my local SPCA. As a temporary solution, I had him in an old rabbit cage, but it was pretty obvious that it wasn't large enough. Hay was frequently getting dumped in the water bowl, and Oreo was constantly moving things around so that he could stretch out at the perfect angle.
Here's the wooden, bi-level rabbit condo I made him instead. It's large enough for Oreo to stretch out in whatever direction he wants, and it's still light enough for me to carry by myself. It took me a full Saturday afternoon to build, but I like to putter, and was using a hand saw for most of the cuts, so your build time may be quite a bit less.
Overall, the interior dimensions are 24"x30", and each level is 18" high. The recommendations that I found are 1sq ft / pound of rabbit, so this cage exceeds that for mine.
Step 1: Materials & tools
1/2" plywood, for the floors and top (cut at the store into three 24"x30" pieces)
3/8" plywood, for the sides (cut at the store into two 24"x36" pieces and one 30"x36" piece)
1/2"x1-1/2" plain wood, enough to be the vertical support in each corner, approx. 12'
1/2"x2-1/2" plain wood, enough to make a kick plate on each level and the perimeter of your doors, approx. 21'
Piece of wood for the ramp, approx 8" wide, at least 24" long
Hardware cloth (rabbit fencing) with wires spaced 1" apart or less, approx 8 sq ft
Some sort of furniture feet
Drill & assorted screws
Staple gun & staples
Some sort of non-toxic wood finish (I used mineral oil and beeswax)
A suggestion: pre-drill all your holes. The wood I was using had a tendency to split if I didn't.