I made an 8’ long one last weekend. It took perhaps 20 minutes to assemble.
Easy to make and cheap to assemble: 2 straight 1x4 boards 2+ times as long as your project is wide and two pieces of 1/4” plywood 10” by 1/2 the board length. My project piece was about 3.5 feet across, so I went with 8 foot boards.
Cut a triangle out of the ends of the plywood then countersink screws to attach ply to boards. The two triangles make a hole for the router bit to fit through. The base plate of the router is screwed to the two plywood pieces. Bigger opening will allow the collet wrench to work without having to detach the router.
Why use two pieces of plywood? More economical, and makes cutting the router hole easy since 1/2 is cut from each piece.
The sled then rides on a rim of wood around your project. I made a 5” and a 4” rim for my project out of the same plywood half sheet. I also propped the project on top of some plywood sheets to avoid having to cut a third set of rim boards for the final pass.
The sled runs across the project piece and holds the router at a level height above the ground/table.