Replacing bottom seat cushion?

Getting the seat cover on the foam base can be a bit tricky as you need to do the job with hog rings.

If you just want to stiffen up the bottom, what you can do (if it's similar to the S4 seats) is remove the seat base from the frame and remove the foam and upholstery as one piece from the base plate. There should be some big openings in the seat foam under your butt that you can stuff with additional dense foam. This avoids the need to remove the hog rings and pull the upholstery off the foam.

Bear in mind it's been a *long* time since I had the seats apart and I did it on a later Spider, so my memory may not be entirely correct or appropriate for an earlier model. Hopefully somebody else who's done the job more recently can chime in. But if you can do it this way it can save you a lot of work.

How difficult is it to get this repair done?

It is not difficult to get this repair done. Or, do you mean for you to do it. Your professional mechanic is afraid of upholstery, just like your professional upholsterer would tell you a how difficult it is to get a timing belt changed.

A professional upholsterer should be able to make short work of it.

It certainly can be a fair amount of work if you are going to do it yourself. Do some research on upholstery technique and allow plenty of extra time.

good luck

You might consider doing the grunt work yourself (to save money) and having an auto upholstery shop R/R the cushion/seat fabric or leather. Remove the seat(s) from the car and bring them to the upholstery shop. I did that with our Spider. The shop stitched up some split seams for less than $25.

not difficult to recover the new foam assuming your existing leather cover is in good shape. The problem may present itself when you find rotting material where the wires are that you hogring to the foam. Otherwise it is a bit time consuming and difficult to get at some of the rings to r&r.