I got a free 8’h 4’x4’ osb freight box in 2000, cut out doors and windows, sloped and sheathed the top panel, roof, hinged a roost-high viewing flap, stapled on hardware cloth over windows, and stuck some big tree branches in it for roosts. Nearly five years later in often severe weather, it is still ugly but it’s dry and has kept in better shape than I have. It is currently a tool shed below my deck. It’s hurky 2x4 reinforced frame) and on big Harbor Freight hard wheels. I should paint it to match the deck. 
My point is (!), the floor was 1/2” plywood square over the OSB plus linoleum on top. It was great until the mice found us. They ate through the floor along edges and feasted on the linoleum. Putting cheap ceramic tiles over the chew site and inviting the neighbor cats reduced mouse ingress. (And yes, I did clean the bedding regularly.) Maybe hardware cloth wrapped over the underside would help? But osb walls worked for me.
I now use a treadle feeder in the plywood chicken sled with marginally improved results. Keeps the mice out of the feed, but does not improve my guineas’ table manners or trigger their killer instinct.
My point is (!), the floor was 1/2” plywood square over the OSB plus linoleum on top. It was great until the mice found us. They ate through the floor along edges and feasted on the linoleum. Putting cheap ceramic tiles over the chew site and inviting the neighbor cats reduced mouse ingress. (And yes, I did clean the bedding regularly.) Maybe hardware cloth wrapped over the underside would help? But osb walls worked for me.
I now use a treadle feeder in the plywood chicken sled with marginally improved results. Keeps the mice out of the feed, but does not improve my guineas’ table manners or trigger their killer instinct.
