I have a Reynolds Solitude wheelset (10-speed Shimano) that I need to modify to use on an 11-speed bike. Reynolds sells an 11-speed freehub body that serves this purpose, but the price is $110. Is there anything else out there that would do the same thing for less? Thanks.

I am pretty sure you can use the newest ultergra 8000 series with a 10 speed hub HG800 11-34 cassette
I have not done this myself, but have seen it posted several times and a quick search suggest that it will work

There is definitely no less expensive way to make the freehub body 1.8mm longer than to buy the freehub from Reynolds. You can do the HG800 cassette as mentioned without modification. There are some companies that will machine the back of an 11 speed cassette 1.8mm. I'd recommend just buying the freehub or just using some other 11 speed wheels.

I have the same Solitude wheels on an old bike and was also thinking of getting the 11 speed hub so I could use them as a backup wheel set for my normal ride. $110 seems a little pricey but sounds like the best solution.

you can also just file/dremel the base of the freehub, that is the cheapest option.