Hi ATLTiger,
Welcome to the Mattress Forum!
Thank you for the compliment, and congratulations on your new mattress! :cheer: You certainly chose something using good quality componentry.
Nest Bedding recommends a foundation system using slats that are not flexible and no more than 4" between the slats. There is more information about appropriate foundations for modern one-sided mattresses, and some of the site members who offer such foundations, in this post.
You would use two twin extra long foundations for a king size, and your least expensive option would probably be some sort of a metal wire grid network, but many of those will have larger gaps between the wire grids. A wood platform foundation certainly can work well, but within your budget range you may be challenged to find something that works with proper slat spacing. You’ll have to do some searching.
The only way that this would firm up a mattress is if you are currently placing your mattress upon a flexing support system, and then transition to a firmer non-flexing system. Otherwise, you may need to switch to the firmer version of the hybrid mattress.
Phoenix