There are a couple of easy fixes. Both of which I've tried and both work.
1) Buy some slab gel, take off the cover, slip the slab gel UNDER the existing foam, mark and cut to size and put everything back together, it needs to be tapered some near the edges and in a little so the old foam will pull down around it. That's pretty good, it's not immediately comfortable but it's great after 15 minutes and good for all day rides and you can't tell it from a stock seat until you've sat on it. The only downside is that if it's very cold and it hasn't been sat on for a while it feels like a couple of bricks until it warms (~15 minutes). You can also make it two slabs with a gap down the center to ease the pressure on the nether regions. You can stuff some gel offcuts or foam offcuts (most places that sell foam have those) into the gaps in the seat which helps with shape.
2) Easier, a tad uglier. Sit & Fly seat cover. Gel waffle seat pad under that, between seat and cover. That's more comfortable than the first option, far easier but it doesn't look as good and the waffle pad will eventually need replacing. (~$20 every 3 years) plus it lifts the effective seat height. Not as much as you'd think, but some.
Slab gel can be found on eBay here as can the alternative components.
Option 1. Personally I'd suggest practice with the old seat cover to get the fit with the gel right and either buy a HF air-stapler and stainless steel staples OR take the new seat cover + bits to an upholstery shop to have them fit it. I've done 1) to two DL650's and my wife's CTX700. Worked fine on all. But 2 is easier and probably cheaper. 2 was not an option on the CTX because of the way the Honda seat fits.
Only did 2 on my DL1000 and it works better. My DL 650 also has a Sit & Fly + gel as well as the underseat gel. The original foam sacked out which is why it got the rebuild +gel, I discovered trick 2 later and why wouldn't you.