In order to attach the back and top to the sides you have to sand the kerfing to match their radius. As you recall, the radius of the plates is determined by the underside arc of the bracing. To do that, I purchased preradiused jigs from LMI at are only 16" across. The length of the body is 20" though and I wasn't willing to pay $75 for a radius dish - let alone $150 for two of them - so I had to get creative.
In my basement I have a 2X4, a chop saw and a bandsaw. I also have a pencil and some rules but that should be a given. Long story short, I cut two 2 foot long sections of the 2x4 and marked the centerline of each as well as the centerline of both the 15' and 30' radius jigs. I traced the arc of each on a 2x4, extended the arc by overlapping the jig over the drawn line and voila! a 24" long radius. After an hour or so of slowly and painfully trying to stay true to that line with the bandsaw and planing/scraping/sanding it as true as I could get it I had two sanding cauls for the rims:
For my birthday I got some spending money and I went to Harbor Freight with a 20% off coupon and bought myself a little 8" drill press. I marked the centerpoint and using the biggest drill bit I have I drilled a hole in the middle of each sanding caul. I also marked out the center in the top of my homemade gobar deck/work table and drilled a hole. I clamped the rims and mold to the table top then I searched in vain for a threaded metal rod but ended up using the drill bit to mount the caul over it:
I grant you, it is imprecise and unwieldy but it works well enough as long as I'm diligent. I monitored the depth of the imaginary "dome" by finding the lowest point in the revolution of the caul and wraped the bit with some painter's tape. Using a block plane I smoothed out the high points then I found some 80 grit sanding belts and a stapler and went to work "driving the bus" as John Hall puts it. It's not perfect but I think I did a pretty good job on the back plate. I have yet to set up the 30' radius caul for the topside rim. Doing the back was exhausting enough. Still, even though the back rim is ready I will wait to glue on the top plate first when it's all braced up.