I like those too! But pricey, phew.
Depending on how heavy your dogs are, a single sheet of 1/2" plywood may work for the tops too...plywood fairly light, pretty inexpensive?
( We used to build tunnels, houses, ramps, platforms and all kinds of things for our guinea pigs...but out of corrugated cardboard, so that's a lot easier/cheaper! Building things for Rumo is much more of an endeavor...)
Rumo ~ rescue shepherd/husky mix