I recently built 2 desks for cheap because I moved and me and my GF needed desks. We did them differently so you have some options.

Get a MDF 1/2 inch thick in the size you want

Get something to hold it up, I choose 2 TV stands, my GF choose 2 cheap plastic 3 drawer cabinet from Walmart. Both of these options total about 30 dollars per desk and require no tools.

If your worried about the desktop moving get some wood screws of the proper length 1" or 1-1/4" and screw them into the MDF + Stand or MDF + plastic bracket of the plastic shelf's.

Once nice thing about the plastic shelf from Walmart is you can take off or add additional shelf's because they come apart. My GF wanted a taller desk and she happen to have the same plastic shelf (we had a total of 3) and topped off to make them 4 shelf's high and now it's the perfect height.

For the TV stands you can simply get a 2x4 and have them cut it to length and frame the MDF or get some other object to raise the desk to the right height.

Make sure the MDF can hold the weight you out on it, but for most purposes it shouldn't sag as long as you keep it's supports just wide enough to slide a person in a chair under and don't load it with bricks or something super heavy in the middle of the desk.