AlienQuilter

Is the foam advertised at Joann's good enough for this?

Do I need to make a thin cover then a thicker one that can be removed for easy cleaning?

Need advice form someone who has done this. This does not need to be "pretty" just thick and comfy.

Even if you have not done this but know of a blog/website where someone has done it and has easy directions I would greatly appreciate it!

The type of fabric would need to take a lot of abuse. He does not want to replace "his chair" yet.

Tartan

Solid foam is expensive so see if you can use a Joann coupon. I would get the foam to fit the chair and then measure for the cover. If it is the square foam with flat sides you can do it a couple of ways. You can measure and cut two squares the same and attach a long piece of fabric all around the sides. OR You can over cut to larger squares and just sew off the corners to make it square like a box bottom bag. I would put either Velcro of a zipper on the back of the pillow for cleaning.

Stitchnripper

I just googled "free cushion patterns" and there were too many to try and cut and paste, so maybe that is something you can try. Also, you might be able to use a pre made cushion and then make a cover for it. Good luck and post the finished product for us to admire!!

EasyPeezy

Don't buy the soft foam as it will just disintegrate. It will feel stiff at first but will get
softer after a little while. I just made two cushions for our kitchen chairs. I used some
foam that I had saved from equipment packaging. Glad I did. I also had to glue them
together with contact cement because I didn't have a piece big enough. Then cut it to
shape. I used a box cutter for cutting the foam. I didn't bother to make a first cover.
If I was making a square cushion, maybe I would have. This was some odd shape...
sort of roundish.
The foam was 1.5" thick so I had to cut some strips for the sides. The Clover binding clips
was a big help when I was constructing the cushions. Best $$ I've spent in a while.
I just used some cotton fabric I had on hand. But you could use some canvas maybe.
There are some nice ones out there.
If you're planning to wash the cover, you might want to preshrink the fabric and put
put a zipper. I hate zippers so I just sewed mine shut. LOL
I used a zig zag stitch plus fray check on the seams.

NJ Quilter

I used their dense foam for my dining room chairs. They are just peachy. My chairs have a decorative front corner cutout. I didn't trust myself enough to cut that well so I made a cardboard template and took the foam pieces to my local upholstry shop and had them cut to the exact shape. Did piping and side piecing and stapled to the bottom board from the chair. They came out really well, if I do say so myself, and have held up well for the last 5-10 years.