There is a particle board product called "sink deck". It is made specifically for laminate work. At one time I believe it was the only substrate that laminate manufacturers would guarantee their products over.
Anyway, that said, as others here have stated 2 piece your deck to size. Now over size the piece of formica by about an inch to allow it to overhang all the way around.
If you need to seam make yourself a mirror jig and using a new bit run it through the 2 pieces of laminate simultaneously. This will create 2 pieces that "mirror" each other and fit perfectly together. Looking at your dimensions though you should be able to get a 5x8 or if you make the 5 foot leg 4'-11" instead then get yourself a 5x5 piece leave it overhanging an inch off the back and run your laminate trimmer around the inside. This will give you the needed overage for trimming afterwards. Be conscious of the overhanging extra stock and use that for your edge trim. Then, trim the edges first, trim them flush and THEN do the top.
To finish off the corner use a laminate file with a down stroke so you don't chip out the corner, again, being cognizant not to scratch the laminate on the face.
If its a small section then you can used tape to hold the seam together when you cement it down.
I would shy away from MDF for pretty much anything. It creates some really nasty dust for one. It has formaldehyde that is released when cut, reeks havoc on your shop, dc, and gets everywhere, and imo is just plain junk.
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