Depends on how high you need the platform to be. If 4-1/4” inches is a good height you can use 2”x4” lumber with a 3/4” piece of plywood for the top. A piece of 2”x4” stood on end is actually 3-1/2” tall. When the 3/4” plywood (actual thickness 23/32”) is added to the top you get your 4-1/4” height. Now just build a 2”x4” frame of the size you need. Put another piece of 2”x4” across the center of the frame and put the plywood on the top. Paint it and you’re ready to go.
Depending on the amount of height you want to add and if you care about the finished look or not you could put the chair frame on cement blocks (they come in many different heights).
If you don't like that idea why not make a frame using 4x4 blocks and attach those to the bottom feet or better yet the frame on the bottom.
Whatever you do please keep safety in mind and make it secure.
They sell chairs with seats that help lift you up. Amazon has a very good selection with reasonable prices too.
For seniors and those with hip injuries, we use a piece of plywood the size of the base of the recliner mounted on top of 2 x 4s. Assemble your box, then use screws to fasten the base of the recliner on top of the box.