I have some large sheets of foamboard (like what real estate signs are made of)- The sheets are 4' x 8' but I need to cut a few inches off the 8 foot side. Is there an easy way to cut the long side straight??? When I tried with razor knife- the cut did not look very good- Any thoughts?

Good day, Recently I undertook the challenge of creating a large Axis and Allies board on two pieces of pine. I have some experience with staining but given the scope of the project I want to get some pro tips. Here's the deal with the board, picture is attached below: The two pieces of pine are attached via hinges, making the total size of the board about 2x3 ft. I used a wood burning tool to do all the lines. I purchased minwax water based stain for the colours and am very happy with the results. The minwax pre-stain is great as it also prevents sharpie from bleeding. I have already done a lot of sharpie work for the writing and it looks great. For my initial poly coat I have a can of waterbased spray and will be doing many light coats to prevent the sharpie from bleeding. Now here's the rub. I really want to add oil based poly for my final coats. I am worried that with all the water based stain, sharpie and water based poly that if someone spills a beer on the board it will reactivate everything and possibly wreck it. I have a can of paint on water based poly as well. I like the colors as they are now so I also don't want to apply anything that will drastically change the color. Some thoughts, insights and ideas on this matter would be helpful. [ATTACH]71719[/ATTACH]