For second levels over wire I have used cheap carpet, foam panels that are meant for kid’s play areas or outdoor paddling pool areas and cheap hardboard wood. All were secured by punching holes in the material and securing with zip ties.
Depends on what your bunny likes to chew and if they have toilet accidents!
Just consider that if he is jumping up and down that the flooring has enough grip. My bunnies seem to care little about comfort and will sleep anywhere on anything! If he still pees outside the box then use something that can be wiped clean like the linoleum.

The minimum recommended cage size for a small-medium sized rabbit is 4’X2′. The dog crate is 2.5’X1.75′ which means it’s about half the recommended minimum and is too small. The minimum is based on floor space and does not include levels. If the crate is bigger than what he’s currently housed then it’s probably worth moving him over temporarily but I would look into finding something more suitable for him and put money towards that instead of a second level.

NIC condo’s are easy to make and relatively inexpensive compared to other housing options.

Overnight is still half the time for the rabbit. I understand you don’t want “pitchforks”, but there are minimum requirements for a reason and everyone on this forum will give you the same opinion if you discuss housing here!
We just love and want the best for all bunnies

How big is the second level? Like if the cage is 2.5′ x 1.75′ is the second level an exact match, or is is more of a base to jump up on (ie. 1′ x 1′); I am only asking because if each level is 12″ high, most bunnies periscope taller than that, and (s)he will need a spot which is the entire height of the dog crate in order to do so.

Anyways, for flooring I would recommend something solid, wood (with linoleum for easy cleaning!) or potentially a plastic chair pad cut to size (remember to shave down corners!) and then something with more traction secured on top (whether a fleece with a rubber grip underneath, or a small rug with rubber backing, etc.) =)

Another suggestion, I’m not sure where you live, but we have Kijiji where I live, and I managed to get an xpen for FREE! (it’s my second one, so I can put 2 together now) – anyways, if you’re able to look for something like that and create a ‘yard’ around the dog crate for extra space, that would be awesome too! I know rabbit cage size is a pain (I no longer have a dining room! ahaha), but even if you only use 3 xpen sides (2 attached to the sides of the dog crate and 1 at the front, it would take up less space than using the full xpen, but still give your bunny another 4 sq ft. of space! =) Just a thought!

Best of luck with floor hunting!! I know it’s a pain (I think I’ve gone through 6 different flooring styles now for my buns)