Usually, a box cutter is good for cutting the corrugated plastic belly covers. The replaceable blade is extremely sharp and can be adjusted for depth (you don't want to cut wires and pipes).
Cut three sides of a rectangle. Leave the side to the front intact. It will act as a hing.
Why not not just drop the entire piece of coroplast?
Canyon
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2017 Ram 1500 Tradesman EcoDiesel
2010 Dutchmen 24 FB-SL
Curt 10,000# WDH
Don't confuse the minor inconvenience of wearing a mask with tyranny or oppression.
I also concur with just dropping the coroplast while maintaining it whole, avoid cutting it up.
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2012 Aspen Trail 2710BH | 280 watts of solar on the roof | 2x6V GC batteries | 100% LED lighting | 1500 & 300 watt PSW inverters | so far strictly boondocking
I would definitely avoid dropping the whole belly cover on my Kodiak Cub 21 ft trailer. It requires removing and replacing hundreds of self drilling and tapping screws into the steel frame. If yours has not been removed previously then you may have to pull nails out of the frame to start with and drill and tap all new holes.
I had a tiny leak. I drilled a 1/4 inch hole in the cover and let the water out. The leak stopped soon after. Definitely the easiest.
On the other hand, if you are willing to make it a big project, drop the whole cover. Dutchmen runs wires and pipes helter-skelter through the under belly. Tie them in bundles and protect them from sharp edges for longer service life.
Insulate the hot water pipes for more hot water at taps. I also insulated and sealed holes and cracks to protect the pipes from freezing.