I'm in the process of raising my driver and passenger seats after installing swivel bases. I've finished the passenger seat and will be doing the driver's seat next.
I fabricated the spacers from 2" x 3" structural steel rectangular tube. I added spacer nuts to the top and bottom of the spacers in order to lift the passenger seat by a total of 3 3/4" in order to facilitate my wife's transfer from her wheel chair. I may only raise the driver's side 3" (or possibly even 2") since going higher may impinge on my leg/steering wheel room.
In any case, it was a fairly easy job once I worked everything out and the structural steel spacers are as strong or stronger than anything in the seat or base. I held off on painting them until I was sure that everything fit OK.
And the bonus -- it gave me an excuse to buy a 7" grinder at HF to cut the steel tube since my 4" grinder just wasn't up to it!
PS. I purchased my swivel bases here: